#include <bits/stdc++.h>
using namespace std;
template <class T>
inline T bigmod(T p, T e, T M) {
  long long ret = 1;
  for (; e > 0; e >>= 1) {
    if (e & 1) ret = (ret * p) % M;
    p = (p * p) % M;
  }
  return (T)ret;
}
template <class T>
inline T gcd(T a, T b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
template <class T>
inline T modinverse(T a, T M) {
  return bigmod(a, M - 2, M);
}
const int N = 3e5 + 505;
const int MOD = 1e9 + 7;
int shift = 100000;
long long pr[N];
long long dp[2][N];
long long calc(int i, int j) {
  if (j < 0) return 0;
  return dp[i][j];
}
int main() {
  int a, b, k, t;
  cin >> a >> b >> k >> t;
  dp[0][0 + shift] = 1;
  int now = 0;
  for (int i = 1; i <= t; i++) {
    now = !now;
    for (int j = -shift; j <= shift; j++) {
      dp[now][j + shift] = calc(now, j + shift - 1) +
                           calc(!now, j + k + shift) -
                           calc(!now, j - k - 1 + shift);
      dp[now][j + shift] = (dp[now][j + shift] % MOD + MOD) % MOD;
    }
  }
  for (int i = shift; i >= -shift; i--) {
    pr[i + shift] = pr[i + 1 + shift] + dp[now][i + shift];
    pr[i + shift] %= MOD;
  }
  long long ans = 0;
  for (int i = -shift; i <= shift; i++) {
    int o = i + b - a + 1;
    ans += (dp[now][i + shift] * pr[max(0, o + shift)]) % MOD;
    ans %= MOD;
  }
  printf("%lld\n", ans);
  return 0;
}
