When did you stop using it?
PowerPC accelerators have been around for the A1200, 4000 and 3000 for a while. OS4 must have a PPC - either one of these or the new AmigaONE.
I think that you may also need a graphics card for OS4. The easiest solution would be to buy an A1. You might be able to get a souped up A4000 or 3000, but it won't be as reliable or powerful.
