Molar mass (formula, molecular mass) is the mass of one mole of a chemical element or a chemical compound. Molar mass is suitable and often used in chemistry because it allows for a light conversion between stoichiometric (molar) relationships represented by a chemical equation and mass relations, which are more significant in practice.
The molar mass of a compound is obtained by the sum of the atomic weights of the atoms which form a compound.
So, formula mass of P4 is:
M(P4) = 4 x Ar(P) = 4 x 31 = 124 g/mole