No more babies?

How do you know/ what made you decide that you didn't want to have any more babies and that your last was your last? 
I have two girls a 4 year old and a 11 weeks old and I still feel like I'd want another baby soon. I mean I thought I wouldn't want more after I had this baby. What if I had another baby and then still wanted another after that? I couldn't see having more than 4 but what "if"