The equalizer only gives you four channel mode as an available option when it think's it's plugged into a car docking sled. Without the car docking sled, there aren't four outputs connected, so, no four channel mode. List of things that the player does differently between home and car mode.

The option to switch between four and two channel mode is somewhere in the equalizer presets screen. It's been a while since I've been in there, so dig for it.

If your player is in the car docking sled but doesn't give you the 2ch/4ch option, it might be having this problem.