The pH scale is not a simple straight-line scale. Each step of one pH unit means the concentration of hydrogen ions has changed by a factor of ten. As the hydrogen ion concentration increases by a factor of ten, the pH decreases by one.
For example, a solution of pH 3 has ten times the hydrogen ion concentration of a solution of pH 4. A solution of pH 2 has ten times the concentration of one at pH 3, and so a hundred times the concentration of one at pH 4. Two pH units apart means a factor of ten times ten, which is 100. Three pH units apart means a factor of 1000.
Going the other way, if you dilute an acid so that the hydrogen ion concentration falls to one tenth of its value, the pH increases by one. A solution at pH 5 has a tenth of the hydrogen ion concentration of a solution at pH 4. Remember that lower pH always means more hydrogen ions.