#include <bits/stdc++.h>
using namespace std;
const long long MOD = (1 ? 100000000000000007 : 233333333333333333);
const int mod = (1 ? 1000000007 : 998244353);
const long long INF = 0x7fffffffffffffff;
const int inf = 0x7fffffff;
const double eps = 1e-9;
const int N = 2e5 + 500;
clock_t start, finish;
void time_in() { start = clock(); }
void time_out() {
  finish = clock();
  double tim = (double)(finish - start) / CLOCKS_PER_SEC;
  printf("Running time is %lf\n", tim);
}
inline long long mul(long long a, long long b) {
  long long s = 0;
  while (b) {
    if (b & 1) s = (s + a) % mod;
    a = (a << 1) % mod;
    b >>= 1;
  }
  return s % mod;
}
inline long long pow_mul(long long a, long long b) {
  long long s = 1;
  while (b) {
    if (b & 1) s = mul(s, a);
    a = mul(a, a);
    b >>= 1;
  }
  return s;
}
inline long long poww(long long a, long long b) {
  long long s = 1;
  while (b) {
    if (b & 1) s = (s * a) % mod;
    a = (a * a) % mod;
    b >>= 1;
  }
  return s % mod;
}
inline int read() {
  char c = getchar();
  int sgn = 1, ret = 0;
  while (c != '-' && (c < '0' || c > '9')) c = getchar();
  if (c == '-') sgn = -1, c = getchar();
  while (c >= '0' && c <= '9') ret = ret * 10 + c - 48, c = getchar();
  return ret * sgn;
}
void write(int x) {
  if (x > 9) write(x / 10);
  putchar(x % 10 + '0');
}
inline long long phi(long long x) {
  long long ans = x;
  for (long long i = 2; i * i <= x; i++)
    if (x % i == 0) {
      ans -= ans / i;
      while (x % i == 0) x /= i;
    }
  if (x > 1) ans -= ans / x;
  return ans;
}
int t, cas;
int m, n;
long long dp[2][N << 1];
int main() {
  int a, b, k;
  while (~scanf("%d%d%d%d", &a, &b, &k, &t)) {
    memset(dp, 0, sizeof dp);
    int len = 200000;
    m = 400500;
    int now = 0;
    dp[now][a - b + len] = 1;
    dp[now][a - b + len + 1] = mod - 1;
    for (int i = 1; i <= t * 2; i++) {
      long long l = 0;
      for (int j = 0; j <= m; j++) {
        l = (l + dp[now][j]) % mod;
        dp[now][j] = 0;
        dp[1 - now][max(j - k, 0)] += l;
        dp[1 - now][max(j - k, 0)] %= mod;
        dp[1 - now][min(j + k + 1, m)] += (mod - l);
        dp[1 - now][min(j + k + 1, m)] %= mod;
      }
      now = 1 - now;
    }
    long long l = 0, ans = 0;
    for (int i = 0; i <= m; i++) {
      l = (l + dp[now][i]) % mod;
      if (i > len) ans = (ans + l) % mod;
    }
    printf("%lld\n", ans);
  }
}
