a(n) = 5*8^n - 1.

Terms

a(0) =4a(1) =39a(2) =319a(3) =2559a(4) =20479a(5) =163839a(6) =1310719a(7) =10485759a(8) =83886079a(9) =671088639a(10) =5368709119a(11) =42949672959a(12) =343597383679