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

Terms

a(0) =1a(1) =4a(2) =36a(3) =144a(4) =1296a(5) =5184a(6) =46656a(7) =186624a(8) =1679616a(9) =6718464a(10) =60466176a(11) =241864704a(12) =2176782336a(13) =8707129344a(14) =78364164096a(15) =313456656384