You will certain gain disk read/write performance boost, but will it really improve overall usage experience? I think no.
You will not get more FPS in the editor. Placing UE4 in a ram disk will improve load times considerably, but you are likely to feel practical benefit of this only for quite large project. Speaking about size, UE4 projects can get quite large.
On the other hand you have greatly increased volatility. Even if you set up a proper incremental back-up system, you are very likely ragequit the idea of having UE4 in ramdisk after first crash between backups.