To determine the pH of a solution with a hydroxide ion concentration of 4, we can use the formula:

$$pH = 14 - pOH$$

Where pOH is the negative logarithm of the hydroxide ion concentration:

$$pOH = -\log[OH^-]$$

Given [OH^-] = 4:

$$pOH = -\log(4) = 0.602$$

Substituting pOH into the pH formula:

$$pH = 14 - 0.602 = 13.398$$

Therefore, the pH of the solution is approximately 13.398.