4 Month Old Chihuahua won't stop trying to bite me?

My 4 month old chihuahua won't ever stop trying to bite me. I know it isn't teething because when i even put my hand near his mouth he lunges forward and trys to chomp on it. If this is normal how can I get him to stop? He won't chew on ANYTHING but my hand. I have bought him many chew toys and even given him a treat everytime he started to chew on it. He's starting to bite kind of hard. and he isn't exactly 4 months old yet, maybe 3 1/2.