#include<bits/stdc++.h>
#define N 200005
#define LL long long
using namespace std;
const int mo=1e9+7;
int n,m,a,b,fc[N],xf[N],ans;
LL fpm(LL x,LL y){ LL s=1; while(y){ if(y&1) s=(s*x)%mo; y>>=1,x=(x*x)%mo;} return s;}
LL C(int n,int m){ return 1LL*fc[n]*xf[m]%mo*xf[n-m]%mo;}
int main()
{
	int i;
	scanf("%d %d %d %d",&n,&m,&a,&b);
	fc[0]=xf[0]=1;
	for(i=1;i<=n+m;i++)
		fc[i]=(1LL*fc[i-1]*i)%mo,xf[i]=fpm(fc[i],mo-2);
	ans=C(n+m-2,n-1);
	for(i=1;i<=b;i++)
		ans=(ans-C(n-a-1+i-1,i-1)*C(a-1+m-i,a-1)%mo+mo)%mo;
	cout<<ans;
	return 0;
}
