A.V.
For various reasons, DD did not start preschool til she was 4. She might've started at 3.5 last winter, but they didn't have a slot. DD's old daycare rolled the kids into preschool by the time they were 4, even if they did "care" before and after school time as well. If he's happy where he is, I would continue with the daycare for another year and then look for a preschool when he's 4, since being in a care environment is what he is already used to. Look for one where they also have a daycare or extended day so that he doesn't have to move back and forth. I was nervous about DD going every day (she does half days, they only do full days 2x month) and she loves it. She cries when they have a holiday or she is too sick to go. So while I don't think preschool is mandatory, I think it would be a good transition for him (find one that is play based) from daycare to school.
My great niece is in daycare only and IMO the difference between a good daycare and a preschool is that a preschool is actually teaching specific lessons, has a schedule, and is gearing toward K vs just keeping them entertained and taken care of. The goals are different.