Not sure if you're asking just about the 4 year old but I assume you are. Be sure you are not expecting more than he can really accomplish at his age. Just saying "Sit still" doesn't work for a child this young for more than a short time; their bodies and minds are too busy. LIke Betsy posted, always have a special activity bag that is only for church hour; have new stuff in it so it's always a surprise for him. Simple things like coloring are fine, maybe even an etch-a-sketch, or church-themed coloring books or even small puzzles he can do right there in his seat, and of course books, which you change weekly. Maybe have a separate "going out" bag with other activities that aren't for church. He needs to be occupied, not told to do something he does not have the capacity yet to do for more than five or 10 minutes at a go. At home, why is he sitting still? Is there a reason? Be sure he has something to DO if you want him to be still (TV does not count!). Do you keep easy craft stuff on hand? Even just paper, crayons, glue, kids' scissors and construction paper, and an easily cleanable, safe place where he can use them on his own and it's OK to be messy there?
Praise him when he IS still, or when he occupies himself nicely like a big boy -- lots of praise will help.
Regarding church time: If there are other wiggly kids in your church, consider starting a "children's church" where parents take turns taking the younger kids out of the sanctuary into a Sunday school room for fun, active things like Bible-themed games and Bible-themed crafts -- there are loads of both on the Internet and at religious bookstores. The kids can stay in worship for the initial prayers and hymns, so they learn what the church service is like and are part of the community, then they go off to children's church when the sermon begins, or at another appropriate time, and return to parents at the service's end. Everyone wins-- adults get to hear the sermon with full attention and kids get activity that is more memorable for them than being forced to sit through a sermon they do not understand.