Well I ended up selling that system, going to get something that will work with my PTZ camera better.
That model would only do: NVR: 2CH@1080P+2CH@720P or 8CH 720P IP Cameras
So by adding my 1080 ptz, I would have been limited to only one other 1080p camera, and 2 720s.. But I would have had to buy 3 other IP cameras and new wiring.. And not have used any of the bnc connectors.
So now the search is on to find a unit that will do 4 x 1080p. if it comes with 4 cams I'll just keep one as a spare and add my ptz.