A boil/lump appeared on my butt-cheek under skin 4 yrs ago n left untreated. nw it has grow to a ping pong ball size n hard. what it could be? m 22yrs

Hard to tell...: ...would need more detail or a picture. given how long it's been there, my best guess is either a sebaceous cyst (not dangerous although at times they can become infected), or a lipoma (a collection of fat cells, not dangerous). you should be evaluated by a doctor if the lump is tender, is draining fluid, and/or is warm to touch. particularly if it is located right next to your anus.