[en] Phase diagram of the q state antiferromagnetic Potts model on a square lattice is investigated in a neighborhood of the critical point h = 4, T = 0. Using a constructive criterion of uniqueness introduced by Dobrushin, Kolafa, and Shlosman, for the antiferromagnetic Ising model, we show that the activity, ac(q) > 1/(q - 1) and the slope increases from zero (q = 2) to infinity (q = is proportional). The phase diagram bulges near the mentioned critical point for 2 < q < is proportional. (author). 5 refs, 6 figs