a(n) = 3*a(n-1) + a(n-2) - 2*a(n-3) (agrees with A019480 for n <= 19 only).

Terms

a(0) =4a(1) =12a(2) =37a(3) =115a(4) =358a(5) =1115a(6) =3473a(7) =10818a(8) =33697a(9) =104963a(10) =326950a(11) =1018419a(12) =3172281a(13) =9881362a(14) =30779529a(15) =95875387a(16) =298642966a(17) =930245227a(18) =2897627873a(19) =9025842914a(20) =28114666161a(21) =87574585651a(22) =272786737286a(23) =849705465187