Unfortunately, you don't get 7.2 AmpHours with 4 1.8 Amphour batteries in parrallel. You only get 1.8 (but at a higher voltage: 5.34 v 1.2). If you wire them in series you will get 7.2, but at only 1.2 volts.
Anyway, I get about four hours of use with backlight on full and intermittent use of WiFi card. My batteries are not as high capacity as yours and are several years old now, so you should get slightly better use time from yours. I also made two packs with AAA batteries. These are about the same capacity as the Z's internal battery, but are very small and easy to cart around. Did you find the instructions for making the battery packs I made? If not, let me know and I'll email or pm it to you.
V-Man