How to model four way pipe fitting.

1.header pipe is 8 inch.
2.one branch is 4 inch, the other is 3 inch.
3.header pipe cross with branch.

Just code the 4 elements framing into the same node - each with the correct properties (diameter & thickness). Note that the piping codes don't recognize this type of fitting, so CAESAR II is going to give you a warning that it can't compute SIFs.