I have an Onkyo 7.1 Receiver. I only want to run 2 Front Speakers, 2 Rear Speakers and a Sub. I think this is a 4.1 system (Four speakers and a sub) - will this work or do I need a front center speaker to get the system to 5.1 and work corrently?

Check this out: eaDHome - What is meant by 5.1, 6.1, and 7.1?

You need a center channel to truly enjoy 5.1 channel. Mostly voices are recorded to the center channel.

The extra 2 channels normally go behind the seating position. Not much material is recorded in 7.1, so the two channels act as dual mono channels, more like 6.1, of which would be DTS-ES.