Is there a minimum gib required for downloading the "pro" version as opposed to basic "core"? Will a 4 gb drive be adequate for both?

Installing Zorin OS
5

Probably, though I have never looked for one. There is a video for pretty much everything on the interwebs...

Zorin OS Pro comes with Installation Support. When you receive your download link for Pro, it comes with the Support Email address you can contact.
Or, scroll to the bottom, here:

EDIT:
Crosslink-

Did you mean RAM or the size of the USB stick?