I'm estimating 118 MB per min of 1280 x 720 HD recording. There are some variables though, like what compression ratio they will use on the iPhone 4, but if it's similar to the 3GS then this should be a pretty close estimate.
When I did these calculations last year for the 3GS my higher end estimate for the compression ratio was 60:1. Using this it would be more like 79 MB / min, or 4.6 GB / hour.