radio channels unlock another channel

[p38maniac 10]

So I have a bit of an odd one here. Im new to scripting and I am making a mission where you have to spot enemy troop locations. Every time a certain target is spotted, it unlocks a radio channel (Radio Alpha, Radio Bravo, etc...). When you use that radio channel it displays a hint saying that the objective is complete. I need a script that when all four of the radio channels have been activated (objectives complete) it unlocks a fifth channel that will end the mission. I was thinking an If Them script where the If is where all four channels have been activated, and the Then is the fifth channel is switched from NULL to being usable. Please help.