#include <iostream>
#include <algorithm>
using namespace std;
using ll = long long;

constexpr int lim = 1e5;
constexpr int mod = 1e9+7;

ll fac[2*(lim+1)];

ll h, w, a, b;

ll pow(ll x, ll n){
  if(n == 0) return 1;
  
  ll tmp = pow(x * x % mod, n / 2);
  return tmp * ((n % 2)? x: 1) % mod;
}

ll mul(ll a, ll b){
  return (a % mod) * (b % mod) % mod;
}

ll comb(ll n, ll r){
  return mul(fac[n], mul(pow(fac[n-r], mod-2), pow(fac[r], mod-2)));
}

int main(){
  cin >> h >> w >> a >> b;

  fac[0] = 1;
  for(int i=1; i<=2*lim; i++) (fac[i] = fac[i-1] * i) %= mod;

  ll ans = 0;
  for(int y=1; y<=h-a; y++){
    ans += (comb(b+y-2, y-1) * comb(h-y+w-b-1, h-y) % mod);
    ans %= mod;
  }

  cout << ans << endl;

  return 0;
}
