Hmmm, sarcoma is cancer, you definitely don't want it to be that. Is it like a tumor, or a broad patch of raised skin?
If it's like a small rubber ball under the skin, it's probably sarcoma. Lots of horses get sarcomas, and live a long happy life with them, but some forms are more aggressive than others. The prognosis isn't great it they show up in a 4 year old. **SORRY!** Some vets remove them, and others let them go.
My guess is that it's actually "sweet itch," which is an allergic reaction that a lot of horses get from being bitten by a certain type of midge (no-see-um).
If it's sweet itch, the horse will likely about bowl you over for scratching it they'll be so happy. It's also strictly seasonal, so it should start to disappear as fall comes around and be completely gone by winter.
You can protect him with SWAT ointment on his belly and lots of fly spray.