#include <bits/stdc++.h>
using namespace std;
const int maxn = 4e5 + 10;
const long long mod = 1e9 + 7;
int a, b, k, t, mx;
long long dp1[2][maxn], dp2[2][maxn], cur1, cur, ans, sm[2][maxn];
int main() {
  scanf("%d%d%d%d", &a, &b, &k, &t);
  cur = 0;
  mx = max(2 * k * t + a, 2 * k * t + b);
  dp1[cur][a + k * t] = 1;
  for (int i = 1; i <= mx; i++) sm[cur][i] = dp1[cur][i] + sm[cur][i - 1];
  for (int i = 1; i <= t; i++) {
    for (int j = 0; j <= mx; j++) {
      dp1[cur ^ 1][j] =
          ((sm[cur][min(mx, j + k)] - sm[cur][max(0, j - k - 1)]) % mod + mod) %
          mod;
      sm[cur ^ 1][j] = dp1[cur ^ 1][j];
      if (j) sm[cur ^ 1][j] = (sm[cur ^ 1][j] + sm[cur ^ 1][j - 1]) % mod;
    }
    memset(dp1[cur], 0, sizeof(dp1[cur]));
    memset(sm[cur], 0, sizeof(sm[cur]));
    cur ^= 1;
  }
  cur1 = cur;
  cur = 0;
  dp2[cur][b + k * t] = 1;
  memset(sm[cur], 0, sizeof(sm[cur]));
  for (int i = 1; i <= mx; i++) sm[cur][i] = dp2[cur][i] + sm[cur][i - 1];
  for (int i = 1; i <= t; i++) {
    for (int j = 0; j <= mx; j++) {
      dp2[cur ^ 1][j] =
          ((sm[cur][min(mx, j + k)] - sm[cur][max(0, j - k - 1)]) % mod + mod) %
          mod;
      sm[cur ^ 1][j] = dp2[cur ^ 1][j];
      if (j) sm[cur ^ 1][j] = (sm[cur ^ 1][j] + sm[cur ^ 1][j - 1]) % mod;
    }
    memset(dp2[cur], 0, sizeof(dp2[cur]));
    memset(sm[cur], 0, sizeof(sm[cur]));
    cur ^= 1;
  }
  for (int i = 1; i <= mx; i++)
    ans = (ans + dp1[cur1][i] * sm[cur][i - 1] % mod) % mod;
  printf("%lld\n", ans);
  return 0;
}
