a(n) = binomial(5*n-1,n).

Terms

a(0) =1a(1) =4a(2) =36a(3) =364a(4) =3876a(5) =42504a(6) =475020a(7) =5379616a(8) =61523748a(9) =708930508a(10) =8217822536a(11) =95722852680