, a necessary and sufficient condition for the solution of the polynomial Pell's equationhas been shown. Also, the polynomial Pell's equation X 2 − DY 2 = 1 has nontrivial solutions X, Y ∈ Q[x] if and only if the values of period of the continued fraction of √ D are 2, 4, 6, 8, 10, 14, 18, and 22 has been shown. In this paper, for the period of the continued fraction of √ D is 4, we show that the polynomial Pell's equation has no nontrivial solutions X, Y ∈ Z[x].