Since I don't have a Sig 4 Star, neither you say which engine you will have installed. I will give you the tools I use to estimate the servos needed for a particular airplane that I'm not familiar with, or the specs are not listed on the manual....
Start with The expected speed of the aircraft:
http://www.rcpro.org/rccalc/pitchspeed.aspx
Then move to the required torque of the servos at the expected speed:
http://www.mnbigbirds.com/Servo%20To...0Caculator.htm
Hope that steers you in the right direction.
Rafael