#include<bits/stdc++.h> 
#define LL long long
#define clr(x,i) memset(x,i,sizeof(x))
using namespace std;
const LL N=200005,mo=1e9+7;
LL n,m,a,b,fac[N+2],inv[N+2];
LL qpow(LL x,LL k)
{
	LL ret=1;
	while(k){
		if(k&1) ret*=x,ret%=mo;
		x*=x;x%=mo;k>>=1;
	}
	return ret;
}
LL C(int x,int y)
{
	return fac[x]*inv[y]%mo*inv[x-y]%mo;
}
int main()
{
	cin>>n>>m>>a>>b;
	fac[0]=1;
	for(int i=1;i<=N;i++) fac[i]=(fac[i-1]*i)%mo;
	inv[N]=qpow(fac[N],mo-2);
	for(int i=N-1;i>=0;i--) inv[i]=(inv[i+1]*(i+1))%mo;
	LL ans=0;
	for(int i=1;i<=n-a;i++){
		LL cur=C(b+i-2,b-1)*C(n-i+m-b-1,m-b-1)%mo;
//		cout<<i<<"  "<<b+i-1<<' '<<b<<' '<<n-i+m-b-1<<' '<<m-b-1<<endl;
//		cout<<C(b+i-1,b)<<' '<<C(n-i+m-b-1,m-b-1)<<endl;
		(ans+=cur)%=mo;
	}
	cout<<ans<<endl;
}

