a(n) = binomial(4n,n).

Terms

a(0) =1a(1) =4a(2) =28a(3) =220a(4) =1820a(5) =15504a(6) =134596a(7) =1184040a(8) =10518300a(9) =94143280a(10) =847660528a(11) =7669339132a(12) =69668534468a(13) =635013559600