Finding molarities of individual reagents given the chemical equation and total concentration in a container
The assumption you need to make is that at time zero, only $\ce{PH3}$ is present.  So relative to the moles at time zero, or $N_0$, you should find $\Delta N = N(t) - N_0$, the total new moles formed by the reaction since it began.

The reaction converts 4 moles of gas to 7.  

So pressure increases mean the reaction is moving to the right.  Every mole of pressure increase must be $\frac{6}{7}$ due to $\ce{H_2}$ and $\frac{1}{7}$ due to $\ce{P4}$ formation.

So just find the total new moles of gas and multiply by $\frac{1}{7}$.