a(n) = 2*a(n-1) + 2*a(n-2) - 4*a(n-3) + a(n-4), where a(0) = 2, a(1) = 4, a(2) = 8, a(3) = 16.

Terms

a(0) =2a(1) =4a(2) =8a(3) =16a(4) =34a(5) =72a(6) =156a(7) =336a(8) =730a(9) =1580a(10) =3432a(11) =7440a(12) =16154a(13) =35040a(14) =76060a(15) =165024a(16) =358162a(17) =777172a(18) =1686632a(19) =3659984a(20) =7942706a(21) =17236024a(22) =37404156a(23) =81169520a(24) =176145962a(25) =382250364a(26) =829518728a(27) =1800123856a(28) =3906429674a(29) =8477282512