a(n) = 2*a(n-1) - a(n-4), where a(0) = 2, a(1) = 4, a(2) = 6, a(3) = 10.

Terms

a(0) =2a(1) =4a(2) =6a(3) =10a(4) =18a(5) =32a(6) =58a(7) =106a(8) =194a(9) =356a(10) =654a(11) =1202a(12) =2210a(13) =4064a(14) =7474a(15) =13746a(16) =25282a(17) =46500a(18) =85526a(19) =157306a(20) =289330a(21) =532160a(22) =978794a(23) =1800282a(24) =3311234a(25) =6090308a(26) =11201822a(27) =20603362a(28) =37895490a(29) =69700672