To find the mole ratio, we must use the coefficients.
This equation is balanced (4 atoms of P and 12 atoms of Br on both sides), so we can go straight to the coefficients.
P₄ doesn't have a coefficient, so 1 is implied
Br₂ has a coefficient of 6
PBr₃ has a coefficient of 4
So according to the coefficients above, for every 1 mole of P₄ that is consumed, 4 moles of PBr₃ are produced. Therefore, the ratio is 1 mole P₄ to 4 moles of PBr₃.
Look at the coefficients in the balanced chemical equation to determine the ratios. For every 1 mole of P₄ consumed (P₄ has a coefficient of 1), 4 moles of PBr₃ would be produced (PBr₃ has a coefficient of 4).