The Hero 4 does work with the H3-3D gimbal. We have a had a few in the shop already that worked just fine. The grinding out of a small piece of the gimbal mount seems to optional as it just barely touches the options button on the side of the Hero 4. As far as FPV goes, there was no problems at all with the testing we have done in the store so far.
Please contact us at email@example.com if you have further questions. Sorry for the delayed response.