#include<iostream>
#include<algorithm>
#include<cstring>
#include<cstdio>
#include<cmath>
using namespace std;
#define ll long long
const int MOD=1e9+7;
const int MAXN=200000;
int h,w,a,b,res,jc[MAXN+10]={1},jcny[MAXN+10]={1};
int quickpow(int x,int y)
{
	int res=1;
	while(y)
	{
		if(y&1)res=(ll)res*x%MOD;
		y>>=1;x=(ll)x*x%MOD;
	}
	return res;
}
int ny(int x){return quickpow(x,MOD-2);}
int C(int n,int m){return (ll)jc[n]*jcny[m]%MOD*jcny[n-m]%MOD;}
int calc(int n,int m){return C(n+m,n);}
int main()
{
	for(int i=1;i<=MAXN;++i)jc[i]=(ll)jc[i-1]*i%MOD,jcny[i]=ny(jc[i]);
	scanf("%d%d%d%d",&h,&w,&a,&b);res=calc(h-1,w-1);
	for(int i=1;i<=b;++i)
	{
		res-=(ll)calc(h-a-1,i-1)*calc(a-1,w-i)%MOD;
		if(res<0)res+=MOD;
	}
	printf("%d",res);
	return 0;
}