4 But seeing that he has changed the order of the words (though he had first said, “O God, Your judgment to the King give Thou, and Your justice to the King's Son,” putting judgment first, then justice), and has put justice first, then judgment, saying, “To judge Your people in justice, and Your poor in judgment:” he does more clearly show that he has called judgment justice, proving that there is no difference made by the order in which the word is placed, because it signifies the same thing.
For it is usual to say “wrong judgment” of that which is unjust: but justice iniquitous or unjust we are not wont to speak of. For if wrong and unjust it be; no longer must it be called justice. Again, by putting down judgment and repeating it under the name of justice, or by putting down justice and repeating it under the name of judgment, he clearly shows that he specially names that judgment which is wont to be put instead of justice, that is, that which cannot be understood of giving an evil judgment.
For in the place where He says, “Judge not according to persons, but right judgment judge ye;” He shows that there may be a wrong judgment, when He says, “right judgment judge ye:” lastly, the one He does forbid, the other He does enjoin. But when without any addition He speaks of judgment, He would at once have just judgment to be understood: as is that which He says, “You forsake the weightier matters of the Law, mercy and judgment.” That also which Jeremiah says is, “making his riches not with judgment.” He says not, making his riches by wrong or unjust judgment, or not with judgment right or just, but not with judgment: calling not anything judgment but what is right and just.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)