In this task, you are given a multiple-choice question about healthcare. Answer the question based on your information and classify your answers into '1', '2', '3', and '4'.

Question: In the determination of water hardness due to the presence of Ca2 + and Mg2 + ions, the titrant reagent is used:
Options: <1> Methyl orange. <2> A standard solution of potassium chloride. <3> Black Eriochrome T. <4> A standard solution of ethylenediaminetetraacetic acid (EDTA). <5> A solution of AEDT previously standardized against a solution of potassium phthalate.
4