I guess it depends on whether the purity is by volume, concentration, or weight. The calculations would be different based on that. (And also whether the 4.25% solution is 4.25% by volume, concentration, or weight.)

Volume percent is defined as volume of solute over volume of solvent. so 4.25mL/100mL is 4.25%. But then it's not really because it's not 100% pure H3PO4. So, I don't know.

Concentration can be quite tricky sometimes if you miss exactly how it's being expressed.