Position of n-th 4 in A031253.

Terms

a(0) =8a(1) =18a(2) =19a(3) =21a(4) =31a(5) =46a(6) =54a(7) =57a(8) =60a(9) =61a(10) =76a(11) =91a(12) =93a(13) =96a(14) =106a(15) =121a(16) =129a(17) =132a(18) =135a(19) =136a(20) =137a(21) =140a(22) =143a(23) =146a(24) =149a(25) =151a(26) =152a(27) =155a(28) =158a(29) =161