4 FullHD Screens on 2 RPi4 running full resolution?

Correct. That’s when using HEVC. Unfortunately playback for that is still somewhat limited. For example it’s not possible to preload the next video while another video is currently running. So there is currently a forced gap between videos. Also HEVC is not 100% stable.

In your case, I’d suggest splitting up the 7680x1080 source video into two anamorphic 1920x1080 H264 videos and play each of the on one device with two attached displays. You will lose half of the horizontal resolution but can use the 100% reliable H264 decoder.

As for the package to use: You can use two HD-player setups, each with one of the videos, enable the synced playback option and assign one to each device. Assuming the duration of both videos is identical, this will result in synced playback.

An alternative is the new, still in development package that also supports synced playback. The advantage is that you can manage both videos within a single setup and assign the same setup to both devices. Each device can then use the ‘alternatives’ feature of that package to play the correct video on its displays.