I agree with the others in that I would not consider this the best way of managing restarts on your endpoints for the purposes of updates.
If you do really want to create repeating scheduled reboots, then perhaps you could just push out a script (via GPO?) to create a scheduled task instead?
http://technet.microsoft.com/en-us/library/cc755618(v=ws.10).aspx
Finally, if you really really want to do this via a HI policy, it should be as simple as creating a poicy with a custom requirement (and no others), and adding the Function -> Utility: Run a program and setting this to perform a shutdown /r /t 0. Then just set this policy to check HI status every 4 hours and you should be away.
Again though, I'd personally go for a managed solution like Altiris instead.