a(n) = 4*a(n-1)-a(n-2)-3*a(n-3)+a(n-4), n>5.

Terms

a(0) =3a(1) =3a(2) =12a(3) =41a(4) =148a(5) =517a(6) =1809a(7) =6316a(8) =22052a(9) =76982a(10) =268737a(11) =938126a(12) =3274873a(13) =11432137a(14) =39908034a(15) =139313506a(16) =486324452a(17) =1697692337a(18) =5926412412a(19) =20688297461a(20) =72220024873a(21) =252110257132a(22) =880082523684