Well you could simply use another trigger to delete the radio trigger as soon as the player group reaches 4.
Name your radio trigger
Triggerone and then set it up however you require to recruit your units.
In the second trigger set it up like this.
none presentcond
count units group player == 4on act
deleteVehicle triggeroneEdit
I've just remembered that some time ago I made something like this but not using the radio.
You just walk up to the unit you want to recruit and when your close enough you get the option to recruit him.
Both the player and the joining unit must be in the trigger area.
http://www.sendspace.com/file/nmg2wfThere are two different triggers you can use.
The one on the left needs the units to have a name so only specific units can be recruited. The unit name must also be added to the trigger list.
The one on the right can be use to recruit any un attached unit to your group as long as both are in the trigger area. No names are needed. It's currently set to West only.