Well, firstly, if the players are actively engaged and responsible, you shouldn't have to put in four hours of work. So, that's a bit of an issue.
But, let's take a standard dungeon crawl. I forget what it's called now, but, I did see a system where the players each contributed a section of the dungeon. As a group. you decide what the dungeon's general theme is, and then each player goes off and builds a section. The DM then takes that and makes changes. Every change the DM makes adds a d4 to a pool (IIIRC, it was 1d4 for a small change up to a 3d4 for a big one) which the players can use to add to any die roll as they proceed through the dungeon.
Poof, ten, fifteen hours of play, 1 hour of prep for the DM. Sure, each player has a pretty good idea what's in the section they designed, but, since the DM has made changes, nothing is for sure.
Now, make that prettier, dress it up with some better language, and away you go.
It's not like these ideas are alien to the hobby. They are there. They just haven't really been incorporated into D&D, mostly because of attitudes like
@Imaro's below. ((See my next post for my response to
@Imaro))