I suggest that you go to the AVS Forum and post on the AUDIO/RECEIVERS forum there.

There are a lot of people there that are into multichannel and multiroom AV stuff and can help out.

Integra produces both receiver and pre/pros. They both support 4 zones. The receiver will prove volume control for each zone. Whether you should go with a pre/pro or a receiver depends on a variety of things including speaker choice. I would look at Integra very seriously. Incidentally, Integra is Onkyo's pro line.

Whether you need IR receivers is dictated on how you will control the multiple zones. If you are going to use traditional remotes in each room, then yes. If you are going to use the App for your iPhone or Android device then you won't. Are you going to install in-wall control units? If you are going to go in that direction, you will need to look at a separate AV controller.

Wheter you use your existing speakers is entirely up to you. If you are going to go in-wall or in-ceiling for some of the speakers, I might consider it for all. They will meld together more effectively.

This is a very advanced install. If you are going to tackle it yourself, commsysman is spot on. You need to get into the AVS forum where people have been through this. My system is 2 zones and no where near as complex as this. I know how I would do it, but that doesn't mean my approach would be very good.