Finding molarities of individual reagents given the chemical equation and total concentration in a container

I am given a table that gives me the different total pressures at certain times. The decomposition of phosphine is the reaction: 


$$\ce{4 PH3(g) <=> P4(g) + 6 H2(g)}$$ 


The time/pressure values are:


\begin{array}{rr}
\mathrm{time\ [min]} & \mathrm{pressure\ [Torr]} \\
\hline
0 & 100 \\
40 & 150\\
80 & 167\\
100 & 172\\
\end{array}


This reaction takes place at 950K. I can then find the molarity of the total gas mixture at each time using $$\frac{n}{V}=\frac{P}{RT}$$


I need to know how to get the pressure of the $\ce{P4}$ for each individual time. How would one go about doing this? 




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 $\frac{1}{7}$.


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