Well, personally, I can never manage to play a game in 4 minutes; I apologize, but I'm just not that quick with the mouse (although I can handle 12 poker tables, pressing all the buttons and still thinking through and evaluating the opponents' status 25 times).
I also think limits like 5000+500 are too high.
In my opinion, the optimal limit is something like 3000+300 or 2000+400. On the one hand, it doesn't allow you to relax and properly encourages you to speed up your thinking, and on the other hand, it's still quite adequate, allowing you to think through the situation and avoid making big mistakes.