How can I convert percent composition by mass to molarity? Specifically, 4% H2O2 to molarity

In a solution of 4% H2O2, there is 4 part H2O2 and 96 part H2O by mass.

Assume 100 gram solution.
Therefore the solution contain = 4/MrH2O2 = 4/34 = 0.118 mol of H2O2.

for water, you can assume the density to be 1 g/cm3 at all condition.
Therefore, 96 gram of H2O equal 96cm3 of water

Therefore, the Molarity is = 0.118/(96/1000) = 1.229M