My baby is always happy and giggly but now hes all fussy. Whats the best thing to help him with teething? Hes only four months old

Teething?????? I'll agree with dr stow that cold works for teething but to me thats a lable tha gets overused. Your ave 4 mo should be smiling and interactive most of the time in spite of teething (some kids never seem bothered). I would be concerned if feeds were off, sleep now different etc. That might mean a treatable problem. The intensity of behavior change might warrant an office visit.
Cold compress. There are many tricks you can try. I personally like taking a wet cloth, freezing it in the freezer and then allow the child to chew on it. This allows them to apply cold directly where it hurts. Watch them of course.
Let them chew. Chewing on something cold and firm is helpful. One idea is to give them a damp washcloth that as been frozen in the freezer to chew on. Commercial teeth rings that can be frozen are available as well. Teething is a normal process. I would avoid medication like painkillers unless absolutely necessary. Most babies really don't need them.
Cold teething ring. The cold is soothing. The teether allows diminution of symptoms. Expect a lot of drooling, chewing, and irritability. Can take months for teeth to finally erupt, with symptoms coming and going. Don't get discouraged.