The 30-year-old started quickly and floored his opponent in the first round after a slick combination.
Rotolo got to his feet and tried to box his way out of trouble, getting through rounds two and three, although it was Barker who was always on the front foot.
In round four the Londoner upped his work-rate again, catching his 37-year-old opponent with some punishing right hands.
Rotolo was clearly dazed and was also suffering a problem with his right hand. His corner pulled him out at the end of the fourth, citing that hand injury.
Barker's victory means that it is two wins from two since his world title defeat against Sergio Martinez, and should put him in the frame for a domestic clash with Andy Lee or Matthew Macklin in the summer.