That's just being pedantic - you could split the existing dump into 4 pieces. It'll run the same either way.
well if the real thing has 4 ROMs, just split it and be done with it. Having one image is technically not the correct way to support the dump.
If cart dumps were done right we wouldnt have the mess we have now with single files for carts that have multiple ROMs in them. For emulation it's fine, but try fixing a busted cart with a single file dump... it's not simple unless you know more than the average Joe about ROM files and splitting/interleaving methods etc. I had a similar problem with Amiga BIOS ROMs. All the ones I found were byte swapped for WinUAE. I wanted to update my A600 to a newer ROM and it didn't work. I eventually figured it out and re-byte-swapped it and it worked fine in the A600. What I'm saying is dumps of ROMs should be supported as they are found on real hardware because it causes trouble later when you need it to fix real hardware. Splitting is all part of the big plan to make MESS conform to MAME standards in a secret deal to rule the emulation world :-D