a fair amount of 4U topics will help solidify 3u concepts..
4u integration makes 3u integration a breeze, same applies with shit like vectors, some proofs (induction), projectile motion stuff etc
overall, most people doing 4u as expected, find related 3u concepts much easier
edit: if you're speaking solely from only doing 3u and doing some side study of 4u concepts, probably wouldn't help a big deal studying 4u topics. it's more the fact that people doing 4u are simply forced to do more study on those topics