a(n) = (11*7^n + 1)/3.

Terms

a(0) =4a(1) =26a(2) =180a(3) =1258a(4) =8804a(5) =61626a(6) =431380a(7) =3019658a(8) =21137604a(9) =147963226a(10) =1035742580a(11) =7250198058a(12) =50751386404a(13) =355259704826