How do you know when you're done?

How do you know when you're done?

I'm 26 & married with 3 kids.... Each of my kids are 4 years apart because it usually takes me a while to decide that I'm ready for another. Well my baby is about 3.5 months old & I kind of want another already! Logically I question myself, mostly because 4 sounds like a lot of kids to me. So tell me, how do you ever know when you're done? How many of you have 4+ kids & what's it like?