Yep, Rayt is right. A4000, A1200 and (I think) A3000 all use 32bit Roms. 500/600/2000/2500 etc all use 16bit Roms. So the A4000 ones won't fit in the A1200 etc. Also, there are physical differences, such as some use 2 x 256 roms, whereas others use 1 x 512.
Here's a basic rundown.
A1000 - 1.0, 1.1, 1.2 (not sure on 1.2)
A500 - 1.2, 1.3, 2.04 in 500+
A2000 - 1.2, 1.3, 2.04
A600 - 2.04
A3000 - 2.04, 3.0
A1200 - 3.0, 3.1
A4000 - 3.0, 3.1
CDTV - Modified 1.3
CD32 - Modified 3.1
These were the kickstarts that were shipped with these models. Newer versions of the kickstart were released for models that didn't ship with them, however.
