This condition of yours is termed as 'fibroid
During pregnancy estrogen
level raises manifold which sometimes increases the size of small unidentifiable mass.
Fibroid can be of minute or to a size of a foot ball.
Size of 4 cm is pretty small. And your baby seems normal by USG scan.
Only when fibroid is obstructing the birth canal normal delivery may not be possible.
Since for you it is small, there is a greater possibility of having normal delivery.
Since you are term there is a less chance of having a preterm delivery.
Bcoz fibroids due to there size may cause preterm labour contractions and delivery. In your case, chances are low.
If the fibroid is on the way of placenta it may cause serious complications, but your report indicates no such thing.
So I believe there is nothing to worry. You will have a normal pregnancy.
Fibroids during pregnancy is usually not treated. Only after delivery it is treated surgically and some non-surgical treatments
is also there which is promising.