How can I set output power of NRF51422 to +4dBm?

[jacky]

Is there any API can I use?

[Steven Wik]

In proprietary mode, change the register for TX power as below:

NRF_RADIO->TXPOWER = (RADIO_TXPOWER_TXPOWER_Pos4dBm << RADIO_TXPOWER_TXPOWER_Pos);

Good examples for how to change the radio registers per the nRF51 RM can be found in the examples folder of the SDK install directory below:

\Nordic Semiconductor\nRF51 SDK_v5.1.0.36092\Nordic\nrf51422\Board\nrf6310\radio_example

Cheers, Steve