a(n) = 4*(10^n - 1)/9.

Terms

a(0) =0a(1) =4a(2) =44a(3) =444a(4) =4444a(5) =44444a(6) =444444a(7) =4444444a(8) =44444444a(9) =444444444a(10) =4444444444a(11) =44444444444a(12) =444444444444