I transfer files to the pi via owncloud, it is incredibly slow on my pi 3b
Just guessing, are you using ethernet and does the Pi 3b have drives plugged in to USB?
A big issue with the Pi 3b is that the ethernet and USB share a single channel, so if you're writing to a USB drive then you're bottlenecking your ethernet and vice versa.
You could try using wifi instead of ethernet. I've squeezed out some extra performance in the past doing so.
To answer your question though, the Pi 4 does remove this limitation so you should see much higher throughput than the 3b