Unfortunately it was from a private individual many months ago..
Ack, that's bad. He probably assumed that A1200 roms were identical to A4000 roms and advertised them as such, however some people reported success into making an A4000 "power up" (I didn't say work fine, just power up) with A1200 roms. Weird.
Here's the deal:
The A1200 3.1 roms lack some important support which A4000 absolutely must have.
Not 100% sure on this, but maybe they lack Zorro II/III support, however with PCMCIA support which A4000 can't understand.
To confuse things even further, it's interesting that Commodore on the other hand made 3.1 roms fully compatible between A500/A600/A2000 series. A 3.1 rom for those Amigas carries full Zorro II support, full PCMCIA support (for the A600) and yet successfully manages to provide flawless operation no mater where you insert it, as far as the above mentioned Amiga models go, e.g. it autosenses the model make/environment and adapts!
On the A3000/A4000/A1200 side, they must be genuine roms made for a particular model, and are not interchangeable.