a(n) = 25*6^(n-2), with a(0)=1 and a(1)=4.

Terms

a(0) =1a(1) =4a(2) =25a(3) =150a(4) =900a(5) =5400a(6) =32400a(7) =194400a(8) =1166400a(9) =6998400a(10) =41990400a(11) =251942400a(12) =1511654400a(13) =9069926400a(14) =54419558400a(15) =326517350400