K.P.
Preschool isn't about academics (or it shouldn't be). It's about social learning. If kids pick-up on early literacy concepts (shapes, colors, letters, etc) then that's great! If not, that's okay too. The whole point is to start learning "how to learn" in a group situation. They learn to interact with their peers, negotiate and follow social norms.
There is no harm in pulling back to three mornings a week, as long as he will be engaged in activities at home on the "off days".