You could use a simple alert to change a custom property when a node is down in the 10 minute window prior to after hours and add that custom property as a factor in the advance alert.
Create "suspended" custom prop
Simple alert = from 3:50 to 4:00 if down change custom prop to suspended = true.
After hour advanced alert add condition to "all" group condition, custom prop not = suspended (suspended not = true)
The only part of this I have not tested is transitioning back to the custom prop = false using a node status = up simple alert. The concern is that it may flood your alert log because most nodes will be in up state every time it is run.