I agree wholeheartedly with Laurie A. My son is six and most days he cleans up just fine on his own. But we still have a rogue day here and there where he needs some guidance with the chore to get him through it.
When he was four we did go through the same thing you are talking about. He just flat our refused to clean up, no matter what we seemed to try. So we changed a few things.
I checked on him more frequently. We took breaks and picked up throughout the day.
We didn't move on until the room was picked up. "Sure you can watch Curious George. It comes on in five minutes. Get all the cars in this box first!"
Sometimes our guidance consisted of picking up small portions of the mess at a time. Ok. Pick up five toys before lunch. Then after lunch I'd ask him to pick up two toys. Etc.
Sometimes "projects" were allowed to be left out.
Put the really messy toys out of reach. With my son, he can play cars all day long. But once the hot wheels tracks come out, it's game over. There are tracks everywhere. Pieces get lost under the couch. It's a pain. So the hotwheels tracks are kept at the top of the closet and he has to ask for them. I get a chance to remind him what is expected at clean up. And since they are out of sight, he often "forgets" they are there. When he was four, the legos had to be put up high or I'd find them everywhere!
Limit where he can play. My kids can play in their room all day and one area of our living room. Certain activities like playdough and painting are only allowing in the dining area. Containing where they can play also contains the mess a bit.
We clean up and pick up toys when my hubby calls and says he is coming home from work. After dinner, very few toys come out. It helps a lot not adding the pick up stress to the end of the day.
And NEVER make a threat you can't live with, it is setting yourself up for failure. Instead of saying you'll toss the toys. How about "toy timeout". This is what happens in our house. If they toys don't get picked up in a reasonable time. They go in time out. On top of the fridge for all to see. Not hidden away in a bag to be forgotten. They get them back after a week. Keeping them on the fridge is a constant reminder and when they ask, you can remind your child over and over that's what happens when toys don't get picked up promptly. NOW...here's the trick. The first few times, you can pick up the time out toys. Then your son will need too. We just added time if our children refused. Otherwise, it may be like our daughter. "It's ok mommy. I don't want to pick up, just put them in time out." So nip that in the bud before he thinks of it on his own.