Now there is a 8GB Raspberry Pi 4 available.
The extra memory should improve disk access, since all free RAM is used for disk caches. You are not likely to notice any improvements when copying very large files, reading or writing. But possibly when copying many small files and during normal use. Also it may make running several large docker images easier.
I suspect that the biggest benefit from the extra RAM is when the RPi4 is used as a desktop computer for light office use and surfing. Possibly together with a SSD.
I bought two, together with a FLIRC case. I will use one for OMV and one for testing as a desktop. I may be able to free up an old HP Elitedesk g3 800 i5 mini. Might be nice to run OMV on, as well. Plenty of USB3. Previous tests with the 4GB model as a desktop was a fail. But a close one.