I am writing my own review as well, wanting to help people who need it for underwater shots and more.
I bought it around mid-summer (2025) and use it mostly for spearfishing. Coming from the AKASO V50X (clearly a more budget-friendly action camera, and I had no expectations from it, so I am not comparing the two cameras), I saw a big difference.
You can feel the build quality as soon as you hold it in your hands. Sturdy construction, and with the case included in the package, it is further reinforced. Also, in this case, the mount with the screw can be attached both at the base for horizontal shots (e.g., 1920x1080, etc.) and on the side for vertical shots like shorts, reels, etc. (e.g., 1080x1920).
The sound it records out of the water is excellent. Inside the underwater case I got for it (TELESIN), the sound is also very good and clear.
In the sea, the camera (and with the mentioned underwater case) does not float and may negatively affect the buoyancy of the speargun (if it is mounted on it).
The video quality is very good both for outdoor and underwater shots.
For underwater shots, tests should be done to adapt it to the user's needs. It can record RAW and then do color correction in a suitable program. In auto, it can also record very beautiful shots, but sometimes, depending on the lighting, angle, depth, season, etc., it emphasizes a different color from RGB (Red, Green, Blue), but this can also be improved with the use of an appropriate program.
The batteries offer very good life.
A very important factor is the correct choice of microsd card to use. You can easily see the recommended cards on DJI's site (for example, I chose the Kingston Canvas Go! Plus microSDXC and have been very satisfied).
Overall, the camera has many positives and is highly recommended!
On the negative side, I mention its weight and DJI's mistake not to include a lens cover in the camera package, which is very useful and costs very, very little.
Other negatives can be improved by the user if they want to spend more time and money (e.g., image quality, sound, etc.).
I hope I helped

Regardless of what the... independent! presenters of action cameras say over there on YouTube (depending - not all - on whose company's interests they serve... GoPro, DJI, INSTA360), the DJI Osmo Action 4 has the most natural image during the day compared to other cameras in the same category, as most viewers of these videos admit in their comments. In low light conditions (night), it still has a very good image for an action camera after the INSTA360 ACE PRO, which is slightly better. The sound in all (GoPro Hero 12-13, DJI Osmo 4, INSTA360 ACE PRO) is equally good without differences. Also, the image stabilization is equally good in all of them without differences. The price is similar for all... with the DJI Osmo 4 being slightly cheaper (I found it for 260 euros). Importantly, the DJI Osmo 4 does not have the problem of the GoPro Hero 12 and 13, which shut down when they overheat. Finally, it has the longest battery life of all (102 minutes in 4k) and makes the deepest underwater shot (up to 18m). Considering these as the most essential, I bought this camera and it truly did not disappoint me!
See how the camera performs in low light conditions, as well as the DJI Mini 4K drone https://www.youtube.com/watch?v=FMMBuwgwstA&t=1s

Well, after some market research I did for the GoPro Hero 12 and the DJI Osmo Action 4, I ended up with the Osmo and I don't regret it at all. I bought the camera mainly for underwater shots since I spend 6 months by the sea (spearfishing, snorkeling, diving). I also purchased the protective case for 70 euros, which can go up to 60 meters deep. In my opinion, when you spend that much money, it's better not to take the risk, even if the manufacturer says you can go without a case up to 18 meters. I won't go into many details. There are countless videos on YouTube for someone to see technical specifications, etc.

TEMPERATURE:
I haven't faced any problems with the temperature so far, and I shoot videos in 4K 60fps (I've also tried it in 4K 120fps for over 45 minutes with no issues).

BATTERY:
The battery is satisfactory. Shooting videos and photos (underwater) gave me at least 1 hour+.

MENU - SETTINGS:
The menu is very easy to use, and the camera is set up from the start. From there, one can make minor adjustments according to their taste.

PHOTOS:
I won't talk about this. I will upload some photos with minimal editing.

VIDEOS:
Here, there is a very small drawback regarding lighting. In low light, it needs some work. But it can be resolved if you make the appropriate adjustments from the menu and perhaps later with some video editing software. The stabilization is amazing.

As I mentioned at the beginning, my review concerns only underwater shots. The camera competes with the GoPro Hero 12 on equal terms and at a much lower purchase cost. Amazing, I don't regret a single euro for this one.

*Do a search on YouTube and first see the basic differences between the two cameras and accordingly make your purchase. Don't go with the logic that it's a GoPro and has a name and waste money for no reason

4K video route Kalambaka - Lamia from the new highway:
https://www.youtube.com/watch?v=u79Eh1H21rM

Cinematic video route Ioannina - Kalambaka - Athens via Central Road:
https://www.youtube.com/watch?v=hN9eoOSCkT8

Purchased as an upgrade from the DJI Osmo Action 3 mainly because the OA3 struggled with night videos and lately I do or complete half of my routes during the night hours.

Package contents:
Camera, selfie stick with a 1/4" screw socket at the bottom (lighter and slightly longer than the selfie stick of the Action3), charger with 3 batteries, 2 quick-release adapters (with slightly less powerful magnets than those of the Action3), along with 2 screws, 1 adhesive mount, 1 plastic frame, 1 cable, and 4 anti-slip stickers.

Construction/Software Stability/Overheating:
Sturdy, small, same dimensions as the OA3, with minimal differences in colors and the description of the sensor and lens on the screw-on lens protector. The feel in hand is that it doesn't break as easily and, judging by my experience with the OA3, this is true. In short, if the old good NOKIA made cameras, these would be the OA3 and OA4.
The camera is waterproof up to 18 meters, which I believe is also the top performance for action cameras of this generation.
The magnetic contacts of the quick-release mechanism for both the camera and the clip seem to have less strength. They remain quite strong, but do not require the same force to detach from each other when you press the two hooks of the clip. It still scares me, and I use a special frame from SmallRig to be more sure.
So far, the camera has not frozen even once, although on its maiden trip from Ioannina to Athens via Kalambaka and E65 (https://www.youtube.com/watch?v=u79Eh1H21rM) it was in operation with 3-4 five-minute breaks for battery changes for over 6 hours.
Since I haven't had the chance to use it indoors with little or no airflow, I don't know how it will behave, but on spring routes with the camera outside the car, it hasn't overheated, which is quite logical and expected.
However, I believe that the European model continues to shut down much earlier than the American one due to EU legislation (paradoxically, GoPro and Insta360 fry an egg before shutting down, while DJI is lukewarm when shutting down due to overheating).

Video quality and stabilization day/night and video sound quality:
The larger sensor (compared to the OA3) offers greater clarity in video and superior image quality at all hours. With the OA4, DJI ensures to improve an already successful recipe and does not try to experiment more than necessary, so the maximum resolution is 4K and the maximum frame rate at 4K is 120fps. That is, just as it was in the OA3.
The OA4 offers the widest lens (155 degrees) on the market without the addition of any extra (GoPro, I’m looking at you) and has, I believe, the lowest image distortion among the 3 flagship action cameras of this generation (GoPro Hero 12, DJI Osmo Action 4, Insta360 AcePro).
It has horizon steady to keep the shot level up to 45 degrees and generally excellent video stabilization.
In evening videos, it leaves the GoPro Hero 12 behind, but shows a bit more ghosting compared to the Insta360 AcePro, so I believe it will satisfy everyone except the most demanding of evening videos. Regarding sound quality, I might be wrong, it might just be my impression, but I feel it is slightly inferior to that of the OA3.

Photos: It takes very nice photos at 10MP and has a rudimentary digital zoom. I don't know if it will help you win an award in a photography contest, but if you don't have particularly high demands (epic zooms and I don't know what else), it will definitely meet your needs, while it also has the capability to take RAW photos for even greater freedom in editing.

Timelapse/Hyperlapse: It is very reliable in Timelapse functions (static long shots, e.g., on a tripod) and Hyperlapse (moving long shots, e.g., in a car) and if someone knows about custom settings and editing through special programs, they will certainly produce very commendable evening Timelapses. However, I have not found a specific feature like Nightlapse/Starlapse/Startrails, etc., as the GoPro Hero 12 and Insta360 AcePro have.

Battery life: I did not notice a difference in operating times between OA4 and OA3, but as anyone who has tried the DJI OA4 or OA3 can tell you, these two cameras have by far the best energy management, resulting in over 1.5 hours of continuous video recording at 4K@25.

Mobile/Tablet app: As has been customary in recent years, the first activation of the camera requires a connection to the DJI Mimo app. Comparing DJI Mimo with GoPro Quik and Insta360 app, I notice that it is generally the slowest of the three in startup from the main screen, in loading icons, and in connecting to the camera, but it is very stable in all other aspects and offers a decent live feed from the camera even after disconnecting and reconnecting the app (I can't say the same for GoPro).

In summary: A very commendable camera, compatible with all accessories for DJI Osmo Action 3. The videos are excellent during the day and very good at night. The other functions are useful, the app is reliable, the build is sturdy, and its battery is top-notch among flagship action cameras. A significant drawback, in my personal opinion, is the quick release system, which does not inspire much confidence when used on a moving vehicle when the camera is exposed to air and vibrations, and the lack of additional night functions like Nightlapse, etc. If you are thinking of buying a camera now, it is a very good choice. If you are considering upgrading from Action 3, the only reason is the much better night video.

Since I have been using it extensively lately, you can see more videos on my channel (IF I manage before the Action 5 is released, I might upload a review video): youtube.com/@TravellerChris