I think 100 square feet of run will certainly be sufficient for 4 chickens! I would highly recommend adding obstacles and things as well... roosts, ladders, things they can climb or flutter up to. Also, things they can hide under(a frames, tunnels, etc...
there’s three reasons I say this:
1. It gives your chickens activities and things to explore!
2. It gives them a way to break line of sight with each other. of one chicken turns out to be a bit of a bully, giving the victims things to escape up to or hide under is essential.
3. It adds more space! If you have 100sqft of ground space, and add four 3 foot roosts, you have added 12 feet of space. If you add a 6 rung ladder with 12” rungs leaned against something, you have added 6 feet of occupieable space... get the idea!?
Good luck with your chicken journey!