The numbers on pH scale can not be compared as the pH 4 is two times stronger than the pH 2. From the pH value, concentration of H_{3}O^{+} ions can be calculated, which is them compared the acidic strength of the solution. pH is negative log of hydronium ion.
pH=-log[H_{3}O^{+}]
Thus, from each pH value, the concentration of H_{3}O^{+} ions can be calculated and compared.
