Professors usually don't write recommendation letters to their courses' students. It's not enough to evaluate the students in a couple of months. I use my master thesis's supervisor as a reference. I agree, I include only people who I'm sure will write good things. Now, I'm lucky in that, because I can say that all my committee members and supervisors were nice. Sometimes, I think I don't deserve all of this niceness, and I don't know how to express my gratitude outwardly, but inside me I'm hugely grateful. If 4 all you got, put them and apply. Don't let it hold you back. Maybe the number isn't a strict requirement.