#include <bits/stdc++.h>
using namespace std;
const int N = 434567;
const int B = 211001;
const int T = 123;
const int MOD = 1e9 + 7;
int pref[N], f[T][N], a, b, k, t;
int main() {
  ios::sync_with_stdio(false);
  cin >> a >> b >> k >> t;
  f[0][a - b + B] = 1;
  for (int i = 0; i < 2 * k + 1; i++) {
    pref[i + a - b + B]++;
  }
  for (int i = 1; i <= t; i++) {
    int pyramid = 0;
    for (int j = -2 * k; j <= 2 * k; j++) {
      pyramid = (((long long)(2 * k - abs(j) + 1) *
                  f[i - 1][-2 * k * i + a - b + j + B] % MOD) +
                 pyramid) %
                MOD;
    }
    for (int j = -2 * k * i + a - b; j <= 2 * k * i + a - b; j++) {
      f[i][j + B] = pyramid;
      pyramid = (pyramid - pref[j + B] + MOD) % MOD;
      pyramid = (pyramid + pref[j + 2 * k + 1 + B]) % MOD;
    }
    for (int j = -2 * k * (i + 1) + a - b; j <= 2 * k * (i + 1) + a - b; j++) {
      pref[j + B] = (pref[j - 1 + B] + f[i][j + B]) % MOD;
    }
    for (int j = 2 * k * (i + 1) + a - b; j >= -2 * k * (i + 1) + a - b; j--) {
      pref[j + B] = (pref[j + B] - pref[j - 2 * k - 1 + B] + MOD) % MOD;
    }
  }
  int sum = 0;
  for (int i = B + 1; i < N; i++) {
    sum = (sum + f[t][i]) % MOD;
  }
  printf("%d\n", sum);
  return 0;
}
