All power settings, and as well the views in the 'non debugging commands' in Aruba OS and Aruba Instant are in EIRP, where the antenna gain is taken into account. So if you have an AP with 14 dBi antenna system (internal/external), if the displayed value is 20 dBm, the transmitter output is 6 dBm (20 EIRP - 14 gain), which is indeed 4mW.
If you set the radio to 0 dBm, with this 14 dBi gain, your EIRP will be 14 dBm.
Note that the setting in your screenshot sets the output power fixed, so it will not change; in the (default) option 'Adaptive radio management (ARM) assigned', you can set the channel and the Maximum power (EIRP) that ARM will use. If ARM detects lots of co-channel interference (neighboring APs on the same channel), it will pick a lower value than the maximum.
Does that answer your question??