There is a cheat that you can do to get an extra position. The cheat is that you get it by using failsafe when you switch your transmitter off.
Gotchas:
Be careful in that most transmitters won't transmit if switches are set strangely or throttle is non-zero when you power on. Think about that. To get control back, you must have throttle zero and FMS set to #1 when you switch transmitter on. Also be aware that some brand / frequencies store the failsafe info in the transmitter and send this info infrequently. This was more common before 2.4g transmitters. If you switch transmitter on first, delay a little, then receiver, it may be a minute or two before the transmitter sends failsafe again. If you then fly and switch transmitter off before the second failsafe transmission, the receiver never got it...
Given a working 3 position FMS setup, you first set failsafe channel (FC or RC) to force FMS position #3 (say 2000us) with other controls in the middle, then you adjust transmitter high endpoint for the FMS channel to move the POS#3 closer to the center (say to 1666us instead of 2000us). Finally set LP FMS config to 4 positions and make POS#4 = RTB.
Test by watching GCS Input screen. You should see the GCS FMS position move 1,2,3 when the transmitter FMS moves 1,2,3, and go to POS#4 when you switch the transmitter off.