The correct statement is:
The substance with a pH of 2.0 would have 100 times as many hydrogen ions per volume of water.
This is because the pH scale is logarithmic, meaning each whole number change on the scale represents a tenfold change in hydrogen ion concentration. Therefore, a difference of 2 pH units (from 2.0 to 4.0) corresponds to a difference of 10^2, or 100 times the concentration of hydrogen ions.