Position of n-th 4 in A031018.

Terms

a(0) =9a(1) =23a(2) =26a(3) =28a(4) =30a(5) =37a(6) =54a(7) =75a(8) =89a(9) =92a(10) =95a(11) =96a(12) =98a(13) =117a(14) =138a(15) =159a(16) =164a(17) =167a(18) =170a(19) =180a(20) =201a(21) =222a(22) =236a(23) =239a(24) =242a(25) =243a(26) =245a(27) =264a(28) =268a(29) =271