I'd get 8GB, I do have trouble now and then with the 4GB, but only because of the VM (I have to reboot), 8GB will give you a longer lifetime I think (I didn't think you could upgrade the RAM in the mini's any more).
In the memory section, it says it's upgradeable.
And I think this is a link from Apple's own instructions on how to do the upgrade.
Mac mini (Late 2014) Mac mini (Late 2014) does not have user-installable RAM. You can configure the memory in your Mac mini (Late 2014) when you purchase it.
I'd ask over on http://apple.stackexchange.com to get a couple of other opinions to :-)
( the reply link appears after a few minutes)