a(n) = 5*6^n - 1.

Terms

a(0) =4a(1) =29a(2) =179a(3) =1079a(4) =6479a(5) =38879a(6) =233279a(7) =1399679a(8) =8398079a(9) =50388479a(10) =302330879a(11) =1813985279a(12) =10883911679a(13) =65303470079a(14) =391820820479