The DJI Zenmuse "for Phantom 2 only" gimbal line are designed specifically to work plug and play (with a little bit of installation of course) on a non-Vision series Phantom 2, so you're definitely on the right track other gimbals are cheaper but just not worth it imo.
The H3-3D was designed for the Hero 3 series, so it is balanced for a hero3. There are therefore 2 main issues if you want to use a Hero 4:
1) balance. a gimbal is a finely tuned piece of equipment so you'll want to check for "neutral balance" and add counterweight accordingly. this may even be true of a H4-3D, for example I had to balance my Hero 3+ on my H3-3D
2) as you hopefully know, the new settings/wifi button on the Hero 4 sticks out a little from the body, which it did NOT on the Hero 3... so what happens is when you mount the hero 4 on the H3-3D plate that button winds up getting pressed (and held) and turns the wifi on which we DO NOT WANT! some people have found that adding some tape in the right place pushes the gopro enough to not be a problem, if you are mechanically inclined it's really not hard to use a dremel to nick out a little spot on the H3-3D plate so that the button isn't depressed.
So those are the concerns... H3-3D's can be found for the mid-$200 range on places like Ebay, while the H4-3D is going to launch at a $399 price point. The question you (and everyone considering buying right now) have to answer for yourself is, do I want to spend less money and have to do a little tweaking, or just pay the extra for a H4-3D which I shouldn't have to muck with much to make work.
in terms of their functionality being a 3-axis gimbal, they're the same. the H4-3D is just going to be balanced and designed around the Hero4