#include <bits/stdc++.h>
using namespace std;
long long dp[105][200005];
long long x = 1e9 + 7, y;
int main() {
  long long a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q;
  scanf("%lld %lld %lld %lld", &a, &b, &c, &d);
  j = c * d;
  for (e = 0, f = 1; e <= j * 2; e++, f++) {
    if (e <= c * 2)
      dp[1][e] = f % x;
    else
      dp[1][e] = dp[1][e - 1];
  }
  for (e = 2; e <= d; e++) {
    for (f = 0; f <= j * 2; f++) {
      h = max(f - c * 2, 0LL);
      i = f;
      if (h - 1 < 0)
        g = 0;
      else
        g = dp[e - 1][h - 1];
      if (f == 0)
        k = 0;
      else
        k = dp[e][f - 1];
      dp[e][f] = (k + dp[e - 1][i] - g) % x;
    }
  }
  for (e = 0, o = 0; e <= j * 2; e++) {
    l = a + e - 1 - b;
    if (l < 0) continue;
    l = min(l, j * 2);
    if (e == 0)
      n = 0;
    else
      n = dp[d][e - 1];
    m = dp[d][l] * (dp[d][e] - n) % x;
    while (m < 0) m += x;
    while (m >= x) m -= x;
    o = (m + o) % x;
  }
  printf("%lld\n", o % x);
  return 0;
}
