"He thinks it's funny." That sentence says it all. He may have not been "ready", but at 4 years old, he can process that this is the way things are supposed to go, and as a PreK and Kinder teacher with my own children, I can tell you that he should be mortified, upset or at least not laughing.
This tells me that it's become a game or control issue with his parents. Normal for his age- especially if he's very intelligent (I suspect he is).
Instead of playing this game with him, you might try making some new rules. First of all, have a family meeting between parents and son. Talk about the potty rules, and how it's your job to help him learn. Tell him he's not in trouble, you're not angry, it's normal, etc. BUT it's time to work a little harder because he wants to be in control of his potty.
Get him some fresh big boy pants, scrub his potty in preparation for working harder, and revamp any reward system you may have in place (new stickers, etc). AND get him his own roll of paper towels . If he potties on the floor, he will clean it up. YOU WILL NOT HELP anymore. It's his responsibility. Now, I know you're saying he can't get it all properly, and this is true, but he doesn't need to know that- you can go back and clean it when he's not around. Instead, he's taking responsibility for his own actions, and, believe me, it will cease to be funny really quickly.
Emphasize the "responsibility" for his own pottying, but stay calm, supportive and matter of fact. A simple, "Oops, that happens sometimes! Here are the towels!" is sufficient. You aren't trying to shame him, just helping him figure it all out. Don't make a big deal over him cleaning it up, either- it shouldn't be a source of positive reinforcement, a simple "thanks" will do. Of course, when he uses the toilet properly, he should get praise.
I've seen this so many times with my students- especially boys, but you'd be surprised how many girls, too. Unless there are medical or developmental issues, it's usually just a matter of finding a way for them to make the connection to WANTING to train themselves.