The x360kb files from the Super Street Fighter IV thread stickied in the software forum work fine for two players.
I used it on my cab for a while with Castle Crashers before I switched to hacked xbox360 pads.
(and I'm unable to play it with the hacked xbox360 pads because the ONE button I don't have wired up is required to play the game)
I assume it would work just as well for 4 as long as you don't have any duplicate keys used.
Just add 2 more pads to the config file.
Here is the layout they used on the machine they built for shows:

Notice the combinations it shows. Someone mentioned in another thread that you could also cycle items with the start button.
Using the workaround is actually superior to using hacked xbox360 pads.
The game doesn't allow you to change the button mapping if using real xbox360 pads.
With the x360kb config file, you can swap the buttons around however you want.