A028899 Map n = Sum c_i 10^i to a(n) = Sum c_i 4^i.

0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 28, 29, 30, 31, 32, 33