I have a customer using 3 external paging zones (3 separate Bogen 2 wire Amps)Zone1 Zone2 Zone3

They use each zone individually and also use all call

They want to add another set of external speakers out side the building which would be zone 4 for emergency use only

They want to be able to use zone 1-2-3 individually and all call zone 1-2-3

but they also want to be able to include zone 4 on an emergency basis only which will include all 4 zones

can this be done or would this possibly require some additional equipment

Thanks

JD