Article IV of chapter 1 of Part the Second of the Constitution is hereby amended by inserting after the words “and to impose and levy proportional and reasonable assessments, rates and taxes, upon all the inhabitants of, and persons resident, and estates lying, within said Commonwealth” the words: -, except that, in addition to the powers conferred under Articles XLI and XCIX of the Amendments, the general court may classify real property according to its use in no more than four classes and to assess, rate and tax such property differently in the classes so established, but proportionately in the same class, and except that reasonable exemptions may be granted.