#include<bits/stdc++.h>
using namespace std;
#define inf 1e9
#define ll long long 
#define M 1000000007
#define P pair<int,int>
#define FOR(i,m,n) for(int i=m;i<n;i++)
#define rep(i,n) FOR(i,0,n)
const int vx[4] = {0,1,0,-1};
const int vy[4] = {1,0,-1,0};
#define PI 3.14159265

ll h,w,a,b,fact[210000],inv[210000],ans;

ll c(ll n,ll m){
  return (ll)(fact[n+m-2])*inv[n-1]%M*inv[m-1]%M;
}

int main(){

  cin>>h>>w>>a>>b;
  fact[0]=fact[1]=inv[0]=inv[1]=1;

  FOR(i,2,h+w+1){
    inv[i]=(ll)(M-M/i)*inv[M%i]%M;
  }
  FOR(i,2,h+w+1){
    fact[i]=(ll)(fact[i-1])*i%M;
    inv[i]=(ll)(inv[i-1])*inv[i]%M;
  }
  FOR(i,b+1,w+1){
    ans=(ans+(ll)(c(h-a,(ll)i))*c(a,w-i+1))%M;
  }

  cout<<ans<<endl;

  return 0;
}
