The U's hit back from behind twice through Tom Craddock and Harry Worley after Darryl Duffy opened the scoring for the Robins and Keith Lowe put the home side 2-1 up at the break.
But James Constable's first goal since February gave the visitors a second-half advantage and Peter Leven rounded off the scoring with a penalty 10 minutes from time.
"We have conceded four joke goals, absolutely pathetic and we were punished," Yates said. "We were in total control, but credit to them for sticking at it.
"We couldn't defend three straight balls and at this level, that's outrageous. We have some big, strong lads, but we let my goalkeeper (Scott Brown) down and we have to learn from our mistakes.
"He had no chance with the goals, but they had four or five shots all night and scored four times. We lost our way and we didn't work their keeper anywhere near enough."