(7*5^n+1)/2.

Terms

a(0) =4a(1) =18a(2) =88a(3) =438a(4) =2188a(5) =10938a(6) =54688a(7) =273438a(8) =1367188a(9) =6835938a(10) =34179688a(11) =170898438a(12) =854492188a(13) =4272460938a(14) =21362304688a(15) =106811523438a(16) =534057617188