Well my auto update interval is set to the default, but every time I connect the first update will be done, if I remain on-line for 240 minutes it will check again, so I fear this won't work.
I'm on dial-up which complicates it further as when I disconnect, that interval begins again and when I reconnect it checks again. So if you aren't on dial-up this may not be an issue, however unless you leave your computer on 24/7 the interval would I believe reset on reboot.