I'm not the right person to ask. I was really answering Tru2chevy's comment "I don't remember exactly what the difference is, but the monsoon system is setup differently than most stock audio systems, I wanna say that each speaker has it's own individual amp or something like that.". There is one amp and a pile of 2 ohm speakers and one pair of 4 ohm speakers (IIRC).
I think if you replace 2s with 4s they won't be as loud. And it might be possible to damage the amp, although that is getting out of my realm of knowledge. I know mismatched impedances can be bad, I just don't know the extent or details.
I also know that there are a few aftermarket replacements for your monsoon stuff, but I don't know exactly what ones. I'll defer to the 4th gen guys on that.
Try this