Coupling detrmination for coupler

pls, can someone tell me how can we determine the coupling factor vs S-parameters of coupler with 04 ports.

P1(input port) _____ P2 (transmitted port)
P3 (coupled port) _____ P4 (isolated port)

Coupling factor (dB) : - 10 log(P3/P1)

it's : S31 ?

If all is terminated with it's reference impedance (presumably 50R): yes

Thank you for confirmation......

but if one of them is not terminated by 50 Ohms, how can we determine the ports powers ...?