S-parameters evaluation of directional coupler

Hi.
I've got a directional coupler which transmitted and coupled ports are loaded with some LC elements. I have to evaluate S14 between input and isolated port on paper. Does anybody know how to do it ? It doesn't have to be a complete answer, maybe just some hint.

Feed power in port1, use dummy load at port 2 and 3, then test power in port4.
You can use vector network analysis to test port 1 to 4 easily.

I can't use vector network analysis. I have to do it on paper and get a formula for S41 as a result...