I think you're looking for Maxis-replacements. You see, these replacements are CC, but still retain the s1-s4 assignment. So if your s4 sim has a child with a s1 sim, the child will have the replacement (e.g, not fugly) s2-3 (possibly s1 and 4 too, I am not sure). For a good starter -
http://modthesims2.com/showthread.php?t=128208