a(n) = 5^n - 5.

Terms

a(0) =-4a(1) =0a(2) =20a(3) =120a(4) =620a(5) =3120a(6) =15620a(7) =78120a(8) =390620a(9) =1953120a(10) =9765620a(11) =48828120a(12) =244140620a(13) =1220703120a(14) =6103515620a(15) =30517578120a(16) =152587890620a(17) =762939453120