That's 4.5hours screen on time? The screen on bar doesn't look like 4.5hours though! Also WiFi on is always going to result in good battery life.
http://i.imgur.com/O58bF.png
I popped out a few times where I wouldn't have been connected to wifi (does the solid bar just show that wifi is enabled but maybe not connected?) but didn't use the phone much in that time.
My previous Desire HD would have lasted about 16 hours of similar usage.