My 4yr old has a small bump on his gumline right above his front tooth. Should I be concerned?

Maybe. This bump could be anything from a wart, an abcess, to the fact that his adult teeth are not positioned correctly and may be causing the bump.....By this age your child should have seen your dentist at least a few times to have their teeth cleaned, and checked. Make an appt. So he can check it out!
Frenum. If the upper lip is attached to it then it's the muscle attachment. If its the bump between the two front teeth it's the incise paprika where the nerve exit from neither should cause concer unless the frontbterth(adult) don't comevtogenther and stay together then it may need to be removed. But reslly you dentist needs to look at it.