"No" is a complete sentence.

For what it's worth, teach him to give his toys away, like to charities (as long as they are in very good condition) so that he learns to pass it on rather than just throw it out. There is an excellent teaching moment, here.

Sustainability? Environment? FFS, dude, he's four. Teach him first that if he doesn't take care of what he has, it won't be replaced. Then later you can bring him into discussions about what it costs you (including costs to the environment if you wish) to replace items that still have use left in them.

You can try the old "no object permanence" trick. Have him "throw away his toys" in a box, store the box far away, and give them back to him on the Fourth of July, just after he "threw away" his new toys. He'll get the toys clean or dirty depending of if he was nice or not six months ago.

If he believes in Santa tell him that Santa doesn't know how many toys he has, and only brings him the gifts he has earned through his good behavior. If he doesn't believe in Santa, tell him that there's a budget for toys, and he's not getting any more than that. If he doesn't stop whinging, tell him that you will help him donate half his toys to Goodwill, and he can find out what happens come Xmas.

Well, if he does that, he just won't have any toys. He has to learn that's not how it works.