If I read your question correctly, you are asking for the conversion to psi if there are 4 volumes of dissolved CO2 in the beer/bottle? I could get all fancy and write out the ideal gas equation, and calculate a number based on all the constants, but the reality is much more simple. If you have 4 volumes of gas, then you have 4x the number of gas molecules, which means 4x the pressure. So, 4 x 14.7 psi (atmospheric pressure) = 58.8 psi