Mammogram and ultrasound showed a 4.8 cm cyst in left breast that appears to be benign. its been three weeks and now i no longer can feel the cyst at all. is it possible for a cyst that large in size to go away on its own?

Yes, that is not at all unusual. Such cysts often increase and decrease in response to hormone fluctuations with your normal cycles.

What you were describing is the classic fibrocystic change. These are glands that sometimes form small cyst and almost always they go away on their own. That is a sign of something that is not worrisome. Decreasing your caffeine intake may help limit their formation