a(n) = 2*a(n-1) + 2*a(n-3) + a(n-4), a(0) = 1, a(1) = 4, a(2) = 9, a(3) = 20.

Terms

a(0) =1a(1) =4a(2) =9a(3) =20a(4) =49a(5) =120a(6) =289a(7) =696a(8) =1681a(9) =4060a(10) =9801a(11) =23660a(12) =57121a(13) =137904a(14) =332929a(15) =803760a(16) =1940449a(17) =4684660a(18) =11309769a(19) =27304196a(20) =65918161a(21) =159140520a(22) =384199201a(23) =927538920a(24) =2239277041a(25) =5406093004a(26) =13051463049a(27) =31509019100a(28) =76069501249