#include <bits/stdc++.h>
using namespace std;
int C[2][401005];
int qzh[2][401005];
int a, b, k, t;
int main() {
  scanf("%d%d%d%d", &a, &b, &k, &t);
  int Z = 2 * t * k + 3;
  C[0][Z] = 1;
  int cur = 1;
  for (int i = 1; i <= 2 * t; i++) {
    int pre = (cur + 1) % 2;
    memset(C[cur], 0, sizeof(C[cur]));
    memset(qzh[pre], 0, sizeof(pre));
    int tt = i * k;
    for (int j = -2 * k * t; j <= 2 * k * t; j++)
      qzh[pre][j + Z] = (qzh[pre][j - 1 + Z] + C[pre][j + Z]) % 1000000007ll;
    for (int j = -tt; j <= tt; j++) {
      int r = min(j + k, tt);
      int l = max(j - k, -tt);
      int p = (qzh[pre][r + Z] - qzh[pre][j - k - 1 + Z] + 1000000007ll) %
              1000000007ll;
      C[cur][j + Z] += p;
      C[cur][j + Z] %= 1000000007ll;
    }
    if (i == 2 * t) {
      memset(qzh[cur], 0, sizeof(qzh[cur]));
      for (int j = -tt; j <= tt; j++)
        qzh[cur][j + Z] = (qzh[cur][j - 1 + Z] + C[cur][j + Z]) % 1000000007ll;
    }
    cur = pre;
  }
  int yy;
  int p = a - b;
  yy = -p + Z;
  cur = (cur + 1) % 2;
  int ans = 0;
  if (yy <= 2 * k * t + Z)
    ans =
        (qzh[cur][2 * k * t + Z] - qzh[cur][yy] + 1000000007ll) % 1000000007ll;
  printf("%d\n", ans);
  return 0;
}
