We investigate two Stieltjes continued fractions given by the paperfolding sequence and the Rudin-Shapiro sequence. By explicitly describing certain subsequences of the convergents P n (x)/Q n (x) modulo 4, we give the formal power series expansions (modulo 4) of these two continued fractions and prove that they are congruent modulo 4 to algebraic series in Z [[x]]. Therefore, the coefficient sequences of the formal power series expansions are 2-automatic. Write Q n (x) = i≥0 a n,i x i . Then (Q n (x)) n≥0 defines a two-dimensional coefficient sequence (a n,i ) n,i≥0 . We prove that the coefficient sequences (a n,i mod 4) n≥0 introduced by both (Q n (x)) n≥0 and (P n (x)) n≥0 are 2-automatic for all i ≥ 0. Moreover, the pictures of these two dimensional coefficient sequences modulo 4 present a kind of self-similar phenomenon.