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

Terms

a(0) =4a(1) =49a(2) =374a(3) =2499a(4) =15624a(5) =93749a(6) =546874a(7) =3124999a(8) =17578124a(9) =97656249a(10) =537109374a(11) =2929687499a(12) =15869140624a(13) =85449218749a(14) =457763671874