The open source firmware OpenTX that can be flashed on a range of low-cost RC transmitters offers more options and is easier to use than the stock firmwares.
The firmware files can be downloaded using the OpenTX Companion (formerly companion9x) software.
Flash OpenTX on Turnigy 9X
Unfortunately the flashing process for the 9X isn't just USB plug and play but involves opening up the remote.
Alternatively, a programmer board with pogo pins can be purchased here.
Single Channel Mode Switch Setup
Single Channel Mode Switch for OpenTX describes how to map the switches to 1 channel and thus freeing 3 channels.