The requirements (Code equations for stress and the allowable values) are different for B31.4 compared to B31.3. The choice of piping code should be defined in the project specifications - this is not an option to you.

The fact that your system passes some load cases in one Code but the same cases fail in a different Code is not surprising. Analyze your system based on the requirements of the project. For the specified Code, determine the cause of the failure and resolve the issue (just like always).