It doesn't matter if the formula is calculated on 4 or 16 oxygen atoms. The monazite formula contains 4 O, but there are 4 formula units in the unit cell. If vacancies are not expected, the formula can also be normalized to 1 cation.

And more generally - there is no single way to write formulas for minerals and chemicals. Sometimes the simplest form is used, sometimes more emphasis is placed on the crystal structure, etc. The de facto formula only gives the ratios of the number of atoms in the substance.