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

Terms

a(0) =1a(1) =1a(2) =3a(3) =8a(4) =21a(5) =54a(6) =140a(7) =363a(8) =941a(9) =2439a(10) =6322a(11) =16387a(12) =42476a(13) =110100a(14) =285385a(15) =739733a(16) =1917427a(17) =4970072a(18) =12882689a(19) =33392610a(20) =86555408a(21) =224356187a(22) =581543081a(23) =1507390367a(24) =3907235410a(25) =10127760455a(26) =26251689768a(27) =68045765768a(28) =176378217169a(29) =457181650329