Floor((5^n-1)/n).

Terms

a(0) =4a(1) =12a(2) =41a(3) =156a(4) =624a(5) =2604a(6) =11160a(7) =48828a(8) =217013a(9) =976562a(10) =4438920a(11) =20345052a(12) =93900240a(13) =435965401a(14) =2034505208a(15) =9536743164a(16) =44878791360a(17) =211927625868