a(n) = 6*a(n-1) - 8.

Terms

a(0) =4a(1) =16a(2) =88a(3) =520a(4) =3112a(5) =18664a(6) =111976a(7) =671848a(8) =4031080a(9) =24186472a(10) =145118824a(11) =870712936a(12) =5224277608a(13) =31345665640a(14) =188073993832