ohm is a measure of resistance. the lower the ohm value the lower the resistance.

for your application, since you have 2 sets of speakers rated at 4 ohms, just connect it to your amp. channel 1 and 2 for your fronts and 3 and 4 for the rear. Each speaker will get around 55 watts. That should be enough. Dont worry about being underpowered 55 watts is plenty for components and coaxials.

When you buy your sub, just hook it up to channel 5