有一个阶梯有n级,每次可以走1,2,3,4级,请问分别有多少种走法?

F(n) = F(n-1)+ F(n-2) + F(n-3)+ F(N-4) (N>=5)
F(1) = 1;
F(2) = F(1)+1 = 2;
F(3) = F(2) + F(1) + 1 = 4;
F(4) =F(3) + F(2) + F(1) +1 = 8;