I would first, put a very visible mark on the child's diaper before dropping him off at the day care. Use a sharpie pen or something very obvious to identify the diaper he had on when he was taken in in the morning.
If he comes home again in the same diaper then do talk to them about it and tell them he needs changed more often.
If they don't change him then report them. During that 4 hours he should've had at least one diaper change at 11:00 am.
A baby toddler nor anyone for that matter should sit in the same wet diaper for too long. Urine turns to ammonia and can cause infections and rashes and even Urinary Tract Infections.