This is my program.
I'm always  getting WA on test 4. I really don't know why.
If you know, send an E-mail to Sunzheng.david@163.com, or just give a reply.
Thank you very much.

#include <stdio.h>
#include <stdlib.h>
#include <memory.h>

int const dir_x[4] = {0, 1, 0, -1};
int const dir_y[4] = {1, 0, -1, 0};
char const dis_c[4] = {'E', 'S', 'W', 'N'};

typedef long double lld;

bool vis[10][10], found;
int w[20][4][4], max_w[20][4][4][4][4][20], dp[20][300][4][4], link[20][4][4], last[20][300][4][4][4];
char out[500];
int n, size = -1;

void search(int &s_floor, int &s_x, int &s_y, int x, int y, int t){
     vis[x][y] = true;

     for (int i = 0; i < 4; i ++){
         int new_x = x + dir_x[i], new_y = y + dir_y[i];
         if (new_x >= 0 && new_x < 4 && new_y >= 0 && new_y < 4 && !vis[new_x][new_y]){
            int new_w = max_w[s_floor][s_x][s_y][x][y][t] + w[s_floor][new_x][new_y];
            if (new_w > max_w[s_floor][s_x][s_y][new_x][new_y][t + 1]){
               max_w[s_floor][s_x][s_y][new_x][new_y][t + 1] = new_w;
               search(s_floor, s_x, s_y, new_x, new_y, t + 1);
            }
         }
     }

     vis[x][y] = false;
}

void find_way(int &s_floor, int &s_x, int &s_y, int &e_x, int &e_y, int &t_dist, int &t_weight, int x, int y, int dist){
     vis[x][y] = true;
     if (x == e_x && y == e_y && t_weight == max_w[s_floor][s_x][s_y][x][y][dist] && dist == t_dist){
        found = true;
        for (int i = 1; i <= dist / 2; i ++){
            char tmp = out[size + i];
            out[size + i] = out[size + dist - i];
            out[size + dist - i] = tmp;
        }
        size += dist - 1;
     }
     if (found) return;

     for (int i = 0; i < 4; i ++){
         int new_x = x + dir_x[i], new_y = y + dir_y[i];
         if (new_x >= 0 && new_x < 4 && new_y >= 0 && new_y < 4 && !vis[new_x][new_y]){
            int new_w = max_w[s_floor][s_x][s_y][x][y][dist] + w[s_floor][new_x][new_y];
            if (new_w > max_w[s_floor][s_x][s_y][new_x][new_y][dist + 1]){
               max_w[s_floor][s_x][s_y][new_x][new_y][dist + 1] = new_w;
               if (found) return;
               out[size + dist] = dis_c[i];
               find_way(s_floor, s_x, s_y, e_x, e_y, t_dist, t_weight, new_x, new_y, dist + 1);
            }
         }
     }

     vis[x][y] = false;
}

int main(void){
    scanf("%d", &n);
    memset(link, 0, sizeof(link));
    memset(dp, 255, sizeof(dp));
    memset(max_w, 255, sizeof(max_w));

    int s_x, s_y;
    for (int c = 1; c <= n; c ++){
        memset(vis, 0, sizeof(vis));
        for (int i = 0; i < 4; i ++){
            for (int j = 0; j < 4; j ++){
                scanf("%d", &w[c][i][j]);
            }
        }

        for (int i = 0; i < 4; i ++){
            for (int j = 0; j < 4; j ++){
                scanf("%d", &link[c][i][j]);
            }
        }

        for (int i = 0; i < 4; i ++){
            for (int j = 0; j < 4; j ++){
                memset(vis, false, sizeof(vis));
                max_w[c][i][j][i][j][1] = w[c][i][j];
                search(c, i, j, i, j, 1);
            }
        }
    }

    scanf("%d%d", &s_x, &s_y);
    s_x --; s_y --;

    dp[0][0][s_x][s_y] = 0;
    link[0][s_x][s_y] = 1;

    for (int c = 1; c <= n; c ++){
        for (int i = 0; i < 4; i ++){
            for (int j = 0; j < 4; j ++){
                for (int k = 0; k <= c * 16; k ++){
                    if (dp[c - 1][k][i][j] >= 0 && link[c - 1][i][j] > 0){
                       for (int l = 0; l < 4; l ++){
                           for (int m = 0; m < 4; m ++){
                               for (int o = 1; o <= 16; o ++){
                                   if (max_w[c][i][j][l][m][o] == -1) continue;
                                   if (dp[c - 1][k][i][j] + max_w[c][i][j][l][m][o] > dp[c][k + o][l][m]){
                                      dp[c][k + o][l][m] = dp[c - 1][k][i][j] + max_w[c][i][j][l][m][o];
                                      last[c][k + o][l][m][0] = i;
                                      last[c][k + o][l][m][1] = j;
                                      last[c][k + o][l][m][2] = o;
                                      last[c][k + o][l][m][3] = max_w[c][i][j][l][m][o];
                                   }
                               }
                           }
                       }
                    }
                }
            }
        }
    }

    lld ans = -1.0;
    int ans_i, ans_j, ans_w;
    for (int i = 0; i < 4; i ++){
        for (int j = 0; j < 4; j ++){
            for (int k = 1; k < n * 16; k ++){
                if (lld(dp[n][k][i][j]) / lld(k) > ans){
                   ans = lld(dp[n][k][i][j]) / lld(k);
                   ans_i = i;
                   ans_j = j;
                   ans_w = k;
                }
            }
        }
    }

    memset(max_w, 255, sizeof(max_w));

    printf("%.4lf\n", double(ans));
    for (int i = n; i >= 1; i --){
        int final_x = last[i][ans_w][ans_i][ans_j][0];
        int final_y = last[i][ans_w][ans_i][ans_j][1];
        int final_w = last[i][ans_w][ans_i][ans_j][2];
        int weight = last[i][ans_w][ans_i][ans_j][3];

        memset(vis, false, sizeof(vis));
        found = false;
        max_w[i][final_x][final_y][final_x][final_y][1] = w[i][final_x][final_y];
        find_way(i, final_x, final_y, ans_i, ans_j, final_w, weight, final_x, final_y, 1);

        ans_i = final_x;
        ans_j = final_y;
        ans_w -= final_w;
        out[++ size] = 'D';
    }

    printf("%d\n", size);
    for (int i = size - 1; i >= 0; i --){
        printf("%c", out[i]);
    }
    printf("\n");
    system("pause");
}

I'm really sorry that I've made a silly mistake. But it really took me a long time to debug...
Many parts of my program are right. Maybe they can be helpful.