If it is diprotic it has two pKa's, if it is triprotic it has three, if it is tetraprotic it has four and so on.
I guess you mean pH=4, not pKa=4, as pKa is not a property of a solution, but of the substance.
Knowing pH and Ka (pKa) you can calculate relative concentrations of acid and conjugate base, it requires just a simple rearranging of a dissociation constant.