Given the prices of accelerators, it might just be cheaper to get another A2000/3000/4000 to use the Toaster and the Personal TBCs...
Yeah, but you'll get a GIGANTIC performance increase by installing a good accelerator. A 4000 needs an accelerator board with RAM. To the point, I'd almost say it can't really live without it. The stock RAM access on an Amiga 4000 is... how shall we put it? COMPLETELY CRAP.
Of course, you don't really know if the RAM access is the
only thing wrong with your motherboard... (The TBCs don't really use the Amiga for much beyond power, so they're kind of a bad test case.) There may be some other damaged traces that the Zorro III slots need to function correctly. That would be my only worry with saying just get an accelerator. Even then, though, if you're actually going to use the system any, you'll probably want an accelerator with local RAM, regardless of the motherboard you're using.