ethylenediaminetetraacetic acid structure. what is the total charge of ion

The structure of ethylenediaminetetraacetic acid (EDTA) is:

O O
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H-N-CH2-CH2-N-CH2-CH2-N-CH2-CH2-N-CH2-CH2-COOH
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H H

The total charge of EDTA is -4, as it has four carboxylic acid groups which each have a negative charge.