Im about to pick up a used Mac Mini and was wondering if theres a way to (temporarily!) use a G4 Powerbook as the display for the mini? Or, could I use an iMacs monitor for the mini?
Generally, no, not in ways that don't involve spending as much on other hardware as you would have on the display to begin with. However, there are various options to do things like remote desktop onto the Mini so that it could be used without a monitor. That's essentially the same thing, but you wouldn't expect fast screen changes like full real time HD video to work flawlessly. This is one example of software that helps you set it up...
I don't know if/which pieces of software are explicitly designed to help you set up a computer that has never been configured on its own monitor, though.