a(n) = (7*9^n + 1)/2.

Terms

a(0) =4a(1) =32a(2) =284a(3) =2552a(4) =22964a(5) =206672a(6) =1860044a(7) =16740392a(8) =150663524a(9) =1355971712a(10) =12203745404a(11) =109833708632a(12) =988503377684