a(n) = 5*n! - 1.

Terms

a(0) =4a(1) =4a(2) =9a(3) =29a(4) =119a(5) =599a(6) =3599a(7) =25199a(8) =201599a(9) =1814399a(10) =18143999a(11) =199583999a(12) =2395007999a(13) =31135103999a(14) =435891455999