And I can confirm that OMV 5 works perfectly fine on a 8GB RPi4.
I just moved the card and everything else from my old 4GB RPi4 to the new 8GB RPi4, also in a FLIRC case. And it just works, as it should do.
I don't expect to see any great benefits apart from a little better disk performance, especially when dealing with many small files, due to the significantly bigger disk caches.