The three phase four wire system basically has two configurations - the Wye (Y) and Delta (Δ) circuits.
Usually the Wye configuration is used by the electric utility for three phase power. The phase-to-neutral voltage is the same on all the three legs, in wye configuration. This is one of the important advantages of the wye system against delta system.
In a symmetrical three phase four wire wye system, all of the three phase conductors will be having the same voltage to the line neutral. In other words, the voltage between phase conductors is mathematically expressed as √3 times the voltage of phase conductor to neutral.
ie., VP-P = √3 VP-N
The phase to phase voltage provided by the generator here is 208V.
So the phase voltage = 208 / √3
= 208 / 1.732
ie., the phase voltage is 120 V when a generator connected in a four wire system provides 208 V three phase power.