My male cat's nipple is red

He is 4 years old and he was neutered about 4 years ago. I don't know if this is serious or not as nothing like this has ever happened before. He seems completely normal. I just happened to notice it when he was licking himself. Is there something I can/should do or do I have to take him to a vet?

Hmmm.  Have you noticed him licking or chewing on it at all?  That's awfully red.  I think I'd at least give my Vet a call and ask them if there is at least something I should put on it or rinse it with (like a saline solution or something).  It certainly looks sore

My Sebastian was doing this from pain and stress. See: Bleeding Nipple in Male Cat (warning: photos) He still goes at it from time to time when his tummy is bothering him. He has chronic pancreatitis.

I would definitely give your vet a call. If he's going at the nipple from some kind of pain or stress from an underlying medical issue, you'll want to address that sooner than later.