I haven't had any first hand experience with this, but I know others that have. Have you spoken to him about why he doesn't want you to leave? There might be an underlying fear or issue that needs to be addressed. Even at 4 he should be able to give you some sort of idea why he's so afraid for you to leave. Maybe take him out to lunch and talk about it there. Talk about how school is a safe place and yes, it can be scary, but it's also fun. It might take a LOT of hyping school on your part. Get REALLY excited when you talk about school. Your attitude goes a LONG way. Or maybe try to leave him with a friend or family member while you go out to lunch or do some shopping. At first, just for 30 minutes, then when he's comfortable with that, bump it up to an hour, and so on.
Before you leave him, even if it's just to go take a shower or do the dishes, make sure you tell him EXACTLY what's going on. You might even want to consider getting a timer. Say "Mommy's going to go do the dishes. I'll set this timer and when it dings, I'll come right back. While I'm busy, you play in the living room with your toys." That might give him a tangible sense of when you're coming back.
Personally, I think holding him back a year isn't going to do either of you any good. I think a therapist is a really good idea. They will be able to give you some more solutions to the issue.
Good luck!