The computer and its
operating system, the
DMX adapter and its drivers, and the lighting software all have to be compatible. That's easy to do with
current products, but fairly difficult to achieve by piecing older stuff together.
Consider whether you want to risk shows on elderly computers. The iMac G4 is at least 14 years old. Even if you find one that runs, every boot up could be its last. Hard drives more than five years old are extremely unreliable.
Look for a computer that is just a couple of years old, running Windows 7 or 10.
Current software and
DMX adapters should work well with it, and you'll have something you can trust a show to.