When my son gets happy an starts babbeling he starts to get a little foamy at the mouth with lots of saliva. Is this normal for babys at four months?

Drool is the Rule. They drool, blow bubbles, etc.--often they also let lots of secretions pool in their upper airway, breathe right through it, and sound gurgly. This is normal. Lots of people call this increased salivation 'teething', but it is unrelated--it is really a normal phase of development that will happen whether teething are coming tomorrow or six months from now.
Enjoy his freedom. Your son is blessed with an exuberance of the moment, sharing time with your interaction and unfettered by the painful idea we acquire over time called a social conscience.His slobber is normal as will his desire to spread it around his face with his fingers, his willingness to wear spit up clothes or sit for a while in poo.