they're both single-driver designs, the main difference being that the -4s have a bass port. so if you're very sure that you need slightly more bass, the scl4s are your ticket... but if the scl3s are working fine for you, don't fix what isn't broke, because i don't think it's worth the $100 if you've already got the -3s
if you want a bigger improvement that you'll probably notice more, you'll want to get the scl5s. those have dual-driver designs.. think tweeter/woofer with a crossover, in bass cabinet terms.
