Intrigued by the larger sensor on the Action 4, I got one recently and hit the water. Initial results were decidedly mixed on my end.
I dive a lot in the Pacific Northwest and Great Lakes - no well-lit tropical waters here. With the noise reduction feature on any setting above -2 (the lowest available setting), I found the camera struggles mightily in medium-low light settings, with low-contrast textures. Results in frequent, pronounced "ghosting" that makes the footage unusuable in my opinion. I'm using Rocksteady stabilization, so maybe it's a combo of the stabilization and the noise reduction. But I won't film without stabilization, so the noise reduction had to go. The increase in grain was far preferable to the ghosting / shimmering effect.,
Once I turned the noise reduction all the way down, the the ghosting issue disappeared. So that's good.
The remaining beef I have with the camera is that footage will sometimes come out waaaaayyy greener than it appeared in the water. Like neon highligher green. I've had this issue with both a fixed value for WB and AWB. Sometimes the color temperature will noticably hop / oscillate between neon green and not neon green.
Above water footage is fantastic, and I prefer the natural color profile to the Hero 12's natural profile (which is virtually indiscernible from vibrant). If I can get the footage to stop coming out so green, I'll be happy with it. Otherwise I'll be forced to use the GoPro and return the DJI.