I fly a Venus II with an OS 1.20 AX and use the Sullivan Carbon Composite flex rods. They work just fine and will in your Four Star 40. The key is to make sure that they are secured at points inside the fuselage. I presume that if the plane came with these type of rods, the tubes that are inside the fuselage are probably already set to something solid so they don't flex. You can always open up the bottom of the fuselage by carefully removing the covering, applying the appropriate glue to the tubes against the formers within the fuselage, then re-applying the covering. That's pretty much what I;ve done with my Sullivan equipped aircraft.

DS.