PROBLEM: I can't install AR4 because my CDKey is an upgrade key, and the installer wants to find AR3 on the computer..
SOLUTION: The installer can accept the AR3 CD disk as proof that you own it. The original CD release of AR4 caused some problems with disk swapping due to the Windows XP auto-play mechanism, so it might be appropriate to use the following solution:
Copy the 460MB file Setup AccuRender.exe from the AccuRender folder on the CD to your desktop. Then put in the AR3 CD and cancel out of the AR3 install. Then run the 460MB file from the desktop, and just push forward, don't cancel out because of the warnings. Then delete Setup AccuRender.exe