4th guard would be Cisco/Salmons. That's how teams play 8 man rotations - a backup PG, backup SF/PF or PF/C, and backup SG/SF. Lakers for example, started Fisher, Kobe, Artest, Gasol, Bynum. Blake off the bench playing PG unless they go small, Barnes and (noble) Odom, and the rest of the guys don't play very much. You start going beyond an 8 or 9 man rotation and you'll have trouble developing lineup chemistry, unless you're still very strict about rotation times and lineups, which Mr Smart frankly isn't.
Your 4th guard sits on the bench not getting more than 10 mins a game. Problem for us is our official guard platoon contains only 1 player above 6'4/6'5.