a(n) = 5*7^n-1.

Terms

a(0) =4a(1) =34a(2) =244a(3) =1714a(4) =12004a(5) =84034a(6) =588244a(7) =4117714a(8) =28824004a(9) =201768034a(10) =1412376244a(11) =9886633714a(12) =69206436004a(13) =484445052034