a(n) = 5*3^n-1.

Terms

a(0) =4a(1) =14a(2) =44a(3) =134a(4) =404a(5) =1214a(6) =3644a(7) =10934a(8) =32804a(9) =98414a(10) =295244a(11) =885734a(12) =2657204a(13) =7971614a(14) =23914844a(15) =71744534a(16) =215233604a(17) =645700814a(18) =1937102444a(19) =5811307334a(20) =17433922004a(21) =52301766014a(22) =156905298044a(23) =470715894134