#include <bits/stdc++.h>
using namespace std;
vector<int> v, w;
int t, k, a, b;
int M = 1000000007;
int maximo(int p, int q) {
  if (p > q) return p;
  return q;
}
int main() {
  cin >> a >> b >> k >> t;
  int N = 4 * k * t + 2;
  v = w = vector<int>(N, 0);
  v[2 * k * t] = 1;
  for (int g = 0; g < t; g++) {
    for (int i = 0; i < N; i++) w[i] = 0;
    for (int i = k; i < N - k - 1; i++) {
      w[i - k] = (w[i - k] + v[i]) % M;
      w[i + k + 1] = (w[i + k + 1] - v[i]) % M;
    }
    for (int i = 1; i < N; i++) w[i] = (w[i] + w[i - 1]) % M;
    for (int i = 0; i < N; i++) v[i] = 0;
    for (int i = k; i < N - k - 1; i++) {
      v[i - k] = (v[i - k] + w[i]) % M;
      v[i + k + 1] = (v[i + k + 1] - w[i]) % M;
    }
    for (int i = 1; i < N; i++) v[i] = (v[i] + v[i - 1]) % M;
  }
  int solution = 0;
  for (int i = maximo(2 * k * t + b - a + 1, 0); i < N - 1; i++)
    solution = (solution + v[i]) % M;
  while (solution < 0) solution = solution + M;
  while (solution >= M) solution = solution - M;
  cout << solution << endl;
}
