1-dB Gain Compression Point in Amplifier
I'd work it like this: 

The 1 dB compression input power is 13.2 dBm

5 dB below this is 8.2 dBm

You have 4 equal-power inputs, so each must have 1/4 of this much power to avoid compression, 

8.2 dBm - 6 dB = 2.2 dBm

This is the input power you'll be operating at. The output power for each signal will be

2.2 dBm + 15 dB = 17.2 dBm