a(n) = 3*5^n+1.

Terms

a(0) =4a(1) =16a(2) =76a(3) =376a(4) =1876a(5) =9376a(6) =46876a(7) =234376a(8) =1171876a(9) =5859376a(10) =29296876a(11) =146484376a(12) =732421876a(13) =3662109376a(14) =18310546876a(15) =91552734376a(16) =457763671876