#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define fi first
#define se second
#define mp make_pair
#define all(x) (x).begin(), (x).end()
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define repp(i, a, b) for (int i = a; i <= (b); ++i)
#define repr(i, a, b) for (int i = a; i >= (b); --i)
#define bit(n) (1LL << (n))
#define len(x) ((ll)(x).size())
#define debug(var) cout << "[" << #var << "]\n" << var << endl
#define int long long
typedef long long ll;
const int INF = 1001001001001001001ll;
const ll LINF = 1001001001001001001ll;
const int MOD = 1000000007;
const double EPS = 1e-9;

template <typename T>
ostream& operator<<(ostream& s, const vector<T>& v) {
  int len = v.size();
  for (int i = 0; i < len; ++i) {
    s << v[i];
    if (i < len - 1)
      s << ' ';
  }
  return s;
}

template <typename T>
ostream& operator<<(ostream& s, const vector<vector<T>>& vv) {
  int len = vv.size();
  for (int i = 0; i < len; ++i) {
    s << vv[i];
    if (i != len - 1)
      s << '\n';
  }
  return s;
}

template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}

template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}

__attribute__((constructor)) void initial() {
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(15);
}

struct mint {
  ll x;
  mint(ll x = 0) : x((x % MOD + MOD) % MOD) {
  }
  mint& operator+=(const mint a) {
    if ((x += a.x) >= MOD)
      x -= MOD;
    return *this;
  }
  mint& operator-=(const mint a) {
    if ((x += MOD - a.x) >= MOD)
      x -= MOD;
    return *this;
  }
  mint& operator*=(const mint a) {
    (x *= a.x) %= MOD;
    return *this;
  }
  mint operator+(const mint a) const {
    mint res(*this);
    return res += a;
  }
  mint operator-(const mint a) const {
    mint res(*this);
    return res -= a;
  }
  mint operator*(const mint a) const {
    mint res(*this);
    return res *= a;
  }

  /* 変数名.pow(n) */
  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  // 以降はMODが素数のみ
  mint inv() const {
    return pow(MOD - 2);
  }
  mint& operator/=(const mint a) {
    return (*this) *= a.inv();
  }
  mint operator/(const mint a) const {
    mint res(*this);
    return res /= a;
  }
};

ostream& operator<<(ostream& s, const mint a) {
  s << a.x;
  return s;
}

/* a^n (MOD) の計算 O(log n) */
ll modpow(ll a, ll n) {
  ll res = 1;
  while (n > 0) {
    if (n & 1)
      res = res * a % MOD;
    a = a * a % MOD;
    n >>= 1;
  }
  return res;
}

// a^{-1} mod を計算する
ll modinv(ll a) {
  return modpow(a, MOD - 2);
}

const int MAX = 5100000;
long long fac[MAX], finv[MAX], inv[MAX];

/* 前処理 O(n) */
void COMinit() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}

/* 計算 O(1) */
long long COM(int n, int k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}

int comb(int H, int W) {
  H--, W--;
  return COM(H + W, H);
}

signed main() {
  int H, W;
  cin >> H >> W;
  int A, B;
  cin >> A >> B;

  COMinit();

  mint ans = 0;
  rep(i, H - A) {
    mint temp = 1;
    int h = i + 1, w = B;
    temp *= comb(h, w);

    h = H - i, w = W - B;
    temp *= comb(h, w);

    ans += temp;
  }
  cout << ans << endl;
}

Measured memory_bytes: 119808