Chromebooks do not have upgradeable memory, you are prettymuch set with what it comes with out of the box. That said they really don't need much to run. I run an Acer C720 2gb as my like knock around the house computer and 2gb is totally fine with it. You can have a lot of tabs open, watch videos etc.
Non- ARM Intel processor chromebooks run 64 bit chrome, so if you get a 4gb one it can take advantage of it. Really though all 4gb does for you is allow you to have more tabs open, don't forget all it is, is a glorified web browser.
That said I spend a lot more time on my chromebook than my desktop, it does like 85% of everything besides gaming well enough. I really only use my desktop PC if I game, need to edit or encode video, program (which doesn't happen much these days) or if for some reason I need full MS Office programs.