#include <bits/stdc++.h>
using namespace std;
template <typename T>
T abs(T x) {
  return x > 0 ? x : -x;
}
template <typename T>
T sqr(T x) {
  return x * x;
}
template <typename T>
ostream &operator<<(ostream &s, const vector<T> &x) {
  s << "[";
  for (auto it : x) {
    s << it << ", ";
  }
  s << "]";
  return s;
}
template <typename T>
ostream &operator<<(ostream &s, const set<T> &x) {
  s << "{";
  for (auto it : x) {
    s << it << ", ";
  }
  s << "}";
  return s;
}
template <typename U, typename V>
ostream &operator<<(ostream &s, const pair<U, V> &x) {
  s << "(" << x.first << ", " << x.second << ")";
  return s;
}
template <typename T>
bool chmax(T &x, const T &y) {
  if (x < y) {
    x = y;
    return true;
  }
  return false;
}
template <typename T>
bool chmin(T &x, const T &y) {
  if (x > y) {
    x = y;
    return true;
  }
  return false;
}
const int maxn = 3e5;
int a[maxn], sum[maxn];
const long long MOD = 1e9 + 7;
int main() {
  int A, B, k, t;
  cin >> A >> B >> k >> t;
  k = 2 * k + 1;
  a[0] = 1;
  for (int i = 0; i < maxn; i++) {
    sum[i] = 1;
  }
  while (t--) {
    for (int i = 0; i < maxn; i++) {
      a[i] = sum[i];
      if (i >= k) {
        a[i] -= sum[i - k];
      }
      if (a[i] < 0) {
        a[i] += MOD;
      }
    }
    for (int i = 0; i < maxn; i++) {
      if (i == 0) {
        sum[i] = a[i];
      } else {
        sum[i] = a[i] + sum[i - 1];
        if (sum[i] >= MOD) {
          sum[i] -= MOD;
        }
      }
    }
  }
  long long ans = 0;
  for (int i = 0; i < maxn; i++) {
    int j = i - B + A;
    j--;
    if (j >= 0 && j < maxn) {
      ans = (ans + a[i] * 1ll * sum[j]) % MOD;
    }
  }
  cout << ans << "\n";
  return 0;
}
