Mode 4 is encrypted and is specifically for military use
In the CONUS, IFF/SSR is a no-go item - they will not let you get airborne if it's down
We always carried a spare box on exercises. However, if we had problems prior to T/O, we wangled it by explaining it had to remain off to avoid interference to other on-board sensors and would do a confirmation check when airborne
When it still didn't work, they let us continue because we had come a long way to take part - well done VACAPS