To the OP: The speaker you wire into the amp dictates what ohm level you will be running. You cannot simply select 2 or 4 ohms, the sub is either-or. What sub are you planning on hooking up to the amp? Subs will have a "nominal impedance" rating; some are single voice coil, some are dual. Each voice coil will have it's own impedance, measured in ohms. If you have a single 4 ohm voice coil, you will run the amp at 4 ohms; there is no way to change that speaker to make the amp run 2 ohms. If your sub is a dual 4 ohm voice coil, than you can wire the two coils together in one of two ways. In a series configuration, your dual 4 ohm sub would be at a 8 ohm final load into the amp, while a parallel configuration would put your sub at a 2 ohm final load. Most car audio subs are a single 4 ohm speaker, but higher end subs will often be the dual 4 ohm speakers.
As someone else mentioned, your 5 channel amp will allow you to wire four 4-ohm speakers into the 4-channel part, and any sub configuration that does not measure below 2 ohms. You will get more power for the sub(s) at 2 ohms than at 4, but a 4 ohm sub will be slightly better sound quality; the lower the impedance, the lower the signal to noise ratio, meaning more distortion.