So are you saying you own a Fun and it fits well, but you're thinking about switching to a 4fun? If it ain't broken don't fix it. Just kidding, but seriously... I think the difference in weight range depends on what you're using it for, not necessarily what class, but what style. I'm not an expert on the subject or anything, but I think the weight range you're in effects the parting line on your boat. If you're on the heavier end, your boat will sit lower in the water, and will be a little more edgy, but you'll probably be able to throw it around easier for tricks. If you're on the lighter end, your boat will sit higher on the water and may be a bit more stable and thus may be a little better for river running.
It's a good day to be a duck....