Just add 4900 to group 1, and uncheck the box next to the entry.
Add a Hunt Group enable {Group 1} button to 4900.
When 4900 wants to take calls, press the hunt group enable button, the light goes green, it's like you logged into Manager, checked the box, and merged the configuration.
When they are done taking calls, press the button again, the light goes off, it's like the box in the hunt group list has been unchecked.
They never change their extension number, they just log in and out of the group they want to take calls for.