I would check with your school before I bought a new computer... the new Macs work with Exam Soft only because they can run Windows via Bootcamp. Exam Soft still does NOT support the Macintosh opperating system. So, it is possible to take an Exam Soft exam on a new Mac IF you are using windows via bootcamp. Which means that you have to buy Windows...which can be kind of pricey. Bootcamp is a free download right now. HOWEVER--even though it IS possible to take exams on Macs, many law schools just don't allow it. Perhaps because bootcamp is still in its beta stage, but I really don't know. I don't think they really need a reason, they can just tell you they don't support it and then you are out of luck.
I am thinking about getting a Mac myself, so I called several law schools that I had applied to and all of them said that either it was not possible to take exams on a Mac, or it was possible, but they just didn't allow it. I think the best thing for you to do would be to call your law school, talk to the IT desk, or whoever controls laptop exams and find out if they allow students to take exams on Macs that run windows/bootcamp. If they don't, then it's not worth buying a new Mac just to take exams, because you won't be able to do that. It seems like most law students with Macs tend to rent/borrow/or buy cheap PCs to use for exams. Some schools are starting to allow students to take exams on their Macs, but for now, the majority do not allow it. You should also check out this website, it's very helpful:http://www.maclawstudents.com/
ANyway, good luck and let me know what you find out! At this point, I would probably keep the G4 if you like it, because you probably can't take exams on a new Mac anyways. That, or I would buy a cheap PC, and use the Mac to take notes, write papers, everything else.