pH is the potential of hydrogen, pH provides a measure of the concentration of hydrogen ion while pOH scale provides the concentration of OH- ions.
pH denotes the negative logarithm of concentration of hydrogen while pOH denotes the negative logarithm of the concentration OH- ions.
Therefore; in this case the pOH= -log (OH-)
= - log (10^-10)
= 10
If the pOH is 10 then the pH of the solution will be 4