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

Terms

a(0) =4a(1) =4a(2) =16a(3) =52a(4) =188a(5) =684a(6) =2512a(7) =9244a(8) =34052a(9) =125476a(10) =462416a(11) =1704212a(12) =6280892a(13) =23148428a(14) =85314448a(15) =314430012a(16) =1158845444a(17) =4270975556a(18) =15740867728a(19) =58013659124a(20) =213811888828a(21) =788013108844