a(n) = (7*11^n+1)/2.

Terms

a(0) =4a(1) =39a(2) =424a(3) =4659a(4) =51244a(5) =563679a(6) =6200464a(7) =68205099a(8) =750256084a(9) =8252816919a(10) =90780986104a(11) =998590847139