#include <stdio.h>
long long quick_pow(long long a,long long b)
{
	long long ret=1,cur=a;
	while (b)
	{
		if (b&1)
			(ret*=cur)%=1000000007;
		(cur*=cur)%=1000000007;
		b>>=1;
	}
	return ret;
}
long long fac[200001],inv[200001];
long long c(int a,int b)
{
	long long ret=fac[a];
	(ret*=inv[b])%=1000000007;
	(ret*=inv[a-b])%=1000000007;
	return ret;
}
int main()
{
	int h,w,a,b;
	scanf("%d%d%d%d",&h,&w,&a,&b);
	fac[0]=inv[0]=1;
	for (int i=1;i<=200000;++i)
	{
		fac[i]=(fac[i-1]*i)%1000000007;
		inv[i]=(inv[i-1]*quick_pow(i,1000000005))%1000000007;
	}
	long long ans=0;
	for (int i=1;i<=h-a;++i)
	{
		long long tmp=c(i-1+b-1,i-1);
		(tmp*=c(w-(b+1)+h-i,w-(b+1)))%=1000000007;
		(ans+=tmp)%=1000000007;
	}
	printf("%lld",ans);
	return 0;
}
