Theres a few things to consider...
First - make the 6 button setup alligned. While it seems like it woulld be better to match the
lengths of the fingers... its not. I tried it that way, and ruined a panel. You naturally pull the middle finger in a tad... much like how you type on the keyboard. The other way feels like a strain, and is both uncomfortable and confusing.
Next... the 2 end people would have trouble feeling confortable at an angle like that... because the panel dosnt match thier angle - and they would have trouble figuring out which way was up. Also, there would be no place to really rest thier arms... that and the corner digging into thier bellies.
If you make the corners diagnol... it might help... However... that means you cant use the sticks for Dual joystick games in 2 player simultainious games - not that I know more than one of them offhand.
In my panel, I put buttons almost under the sticks... this makes the wrist of the hands closer together, and takes up less space. However, its still probably wider than most panels, and still not sure how comfortable 4 players would be with it. Guess it also depends on the width of the players! : ) heh
You could also go the route of having the 2 extra players control the games with plug in
side- gamepads as well.
Ohh - and dont forget where the wrist rest. The right players wrist might end up interfereing with the left players buttons.

(my 4 player buttons are a tad (maybe 1/2") bit far too under the joystick tho)
Good Luck ^_^