Yeah, i don't understand this 4 hours thing. I've been here more than 4 hours---days even, but apparently only time I spend posting counts toward my hours? So I guess I need to post a lot more....
If you spend too much time idle on a page, it probably won't count towards your total time logged in. I'd guess it's calculated using the intervals between your page views unless the time between two page views reaches a threshold at which they'd consider you idle. I.e., if you read a page called "Check Out My New Rig [PICS]" for 3 minutes and then navigate somewhere else, it will probably count towards your time. If you view a thread called "Litecoin/cgminer Installation Walkthrough" and spend 30min reading huge posts and going through the installation procedure without navigating anywhere else, it probably won't count towards your time logged in. Otherwise, you could just be afk in a thread for 4 hrs and have 4 hrs logged in.