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

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =8a(5) =15a(6) =43a(7) =113a(8) =295a(9) =778a(10) =2045a(11) =5377a(12) =14141a(13) =37185a(14) =97784a(15) =257139a(16) =676187a(17) =1778141a(18) =4675903a(19) =12296026a(20) =32334345a(21) =85028273a(22) =223595289a(23) =587979169a(24) =1546184200a(25) =4065935847a(26) =10692021243a(27) =28116360553a(28) =73936416023a(29) =194427497258