Reread b00mer's post -- at this point it's not so much a matter of whether 8-meg SIMMs will work, but that he can't actually FIT them in the fast RAM sockets; they'll only physically sit in the chip RAM socket.
b00mer -- are there little plastic tabs on the fast RAM sockets? I'm wondering if it's not so much that they don't FIT, but they don't HOLD...know what I mean? The Amiga 4000 motherboard RAM sockets are notorious for being extremely fragile. Got so bad that the SIMM holders on mine actually broke off, and if a SIMM got loose while I was working on something, my Amiga would freeze and refuse to boot. When I had my A4000 housed in a tower, they actually had to hot-glue the SIMMs in place so they wouldn't fall out.
BTW, the trick to fitting SIMMs in is sort of lining up the contacts with the socket, then sort of angling the SIMM back toward the motherboard, and you should hear a click sound.
If all else fails, it may be worth it to drop some money in an accelerator or a RAM expansion card so you can add some fast RAM without even dealing with the motherboard.
