Stephen,
You need to do a Tow Fatigue analysis. Tow Fatigue will analyze fatigue damage of a structure by generating transfer functions using motion RAO's (Velocity, Motion or Acceleration RAO's) and combining those transfer functions with the wave spectra (Jonswap). I don't think you actually need to combine the 4 sets of RAO's as they should simply be 4 different ways to represent the same set of data. However if you do indeed have 4 sets of RAO's they would most likely not be for the same fatigue environment and could be combined linearly using SACS fatigue damage output.
Tow Fatigue Workflow:
You can select the option on the FTOPT2 line to output a fatigue damage file which can then be used as input for a second fatigue analysis. It will always be valid to linearly combine tow fatigue damages from separate fatigue environments (e.g. tow fatigue and in-place wave fatigue since the responses are not coupled in any way). Refer to Section 2.8 of the Tow Manual for more information about specifying RAO's and wave definitions and Section 3.4.1 of the Fatigue Manual for more information on specifying the wave spectra.
Regards,
Geoff
this helped me, thus put a marking here.