a(n) = (2/5)*(9*6^(n-2)+1).

Terms

a(0) =1a(1) =4a(2) =22a(3) =130a(4) =778a(5) =4666a(6) =27994a(7) =167962a(8) =1007770a(9) =6046618a(10) =36279706a(11) =217678234a(12) =1306069402a(13) =7836416410a(14) =47018498458a(15) =282110990746