I would add two pinball buttons (left/right flippers) on the side on the control panel as well as adding individual CoinUp buttons for player1->4.
I have two pause buttons and they come in handy so either player on each side of the board can hit the pause button easily without reaching too far across a 4-player control panel.
My admin buttons include Tab, Tilda, Esc, Enter, Reset
PS: I do not believe you need 'O' and 'K' buttons since moving the joystick of player1 right, then left (or is it the other way around??) will perform the 'O'+'K' function as disired.
You can check out my 4-player control panel layout at my site:
http://unclet.arcadecontrols.comGood Luck...