Sometimes kids outgrow a daycare situation. With the wider age gap that usually exists with them, providers can have a hard time doing the age appropriate activities for BOTH age groups. So usually the age group with the most need infants and young toddlers, get more of the provider's focus.
Does your provider work on any preschool readiness activities with your son? Painting, glueing, cutting with scissors, playdough, tying shoes, numbers, letters, measuring, music, books, puppet shows, dress up etc...? If all he is doing is playing whether inside or out, he's probably bored and that's why you're getting the negative comments from her. At 4 kids need more structure and more stimulation than free play.
I am a big advocate for quality daycare and preschool. I think both are good stepping stones from home to daycare, from daycare to preschool, preschool to kinder. Each have enriching qualities about that that can help kids in each transition.
I would speak to his daycare provider though. Let her know that you're considering placing him in preschool and get her take on it. Chances are she feels he's ready too, and putting him in half days would still keep the familiarity and security of her environment while giving him the stimulation he needs.
Good luck!