a(n) = 3*a(n-1)-3*a(n-2)+a(n-3)+a(n-4).

Terms

a(0) =1a(1) =2a(2) =4a(3) =8a(4) =15a(5) =27a(6) =48a(7) =86a(8) =156a(9) =285a(10) =521a(11) =950a(12) =1728a(13) =3140a(14) =5707a(15) =10379a(16) =18884a(17) =34362a(18) =62520a(19) =113737a(20) =206897a(21) =376362a(22) =684652a(23) =1245504a(24) =2265815a(25) =4121947a(26) =7498552a(27) =13641134a(28) =24815508a(29) =45143621