Position of n-th 4 in A031087.

Terms

a(0) =4a(1) =17a(2) =35a(3) =53a(4) =64a(5) =66a(6) =68a(7) =70a(8) =71a(9) =72a(10) =74a(11) =76a(12) =78a(13) =80a(14) =89a(15) =107a(16) =125a(17) =143a(18) =165a(19) =192a(20) =219a(21) =246a(22) =262a(23) =265a(24) =268a(25) =271a(26) =273a(27) =274a(28) =277a(29) =280