Two things:
1) The program already generates the wind loads for those cases. In fact those cases are really lust a linear super-position of the two directions of the Case 1 or Case 2 loading.
What I mean by that is that case 3 is just case 1 applied in both directions and multipled by 0.75.
2) Prior to version 8.1 of RISA-3D, the Load Combination generator did NOT automatically produce Load Combinations that were consistent with Case 3 and 4. That is what you are really saying. Therefore, the older versions (8.0.3 and earlier) would require the user to add in a load combination that included both WLX and WLZ to get their case 3.
However, version 8.1 did add in this option. We added another check-box to the LC Generator that says "X & Z w/ Quartering winds". I believe that will produce what you are looking for.
Now, it's just a matter of when the majority of the users will get the update.... They are shipping now, so I'd imaging that you'll be getting it soon.
