a(n) = [x^n] (1 + x + x^2 + x^3)^(4*n)/(1 + x + x^2)^(3*n).

Terms

a(0) =1a(1) =1a(2) =3a(3) =19a(4) =67a(5) =251a(6) =1137a(7) =4803a(8) =20035a(9) =87013a(10) =377753a(11) =1634469a(12) =7134385a(13) =31261114a(14) =137121113a(15) =603206144a(16) =2660097603a(17) =11749336328a(18) =51981371895a(19) =230336544210