a(n) = a(n-2) + 2*a(n-3) + a(n-4).

Terms

a(0) =1a(1) =4a(2) =4a(3) =7a(4) =13a(5) =19a(6) =31a(7) =52a(8) =82a(9) =133a(10) =217a(11) =349a(12) =565a(13) =916a(14) =1480a(15) =2395a(16) =3877a(17) =6271a(18) =10147a(19) =16420a(20) =26566a(21) =42985a(22) =69553a(23) =112537a(24) =182089a(25) =294628a(26) =476716a(27) =771343a(28) =1248061a(29) =2019403