For those who think the 4-player game may go on too long. Please post any ideas for 4-player variants here.

An obvious one that comes to mind:

After a player wins their 3rd prize, only the next two players have a chance to steal from them. If after the next two players take their turn the first player still has 3 prizes, they win.

After playing a couple more 4-player games, I'm not sure it needs any tweaking, but here's another idea anyways.

If a player rolls a prize they already own, they may lock the prize by flipping the tile over. That prize may no longer be stolen. This ends the players turn.