#include <bits/stdc++.h>
using namespace std;
#define ii pair<int,int>
#define ll long long
#define all(x) (x).begin(),(x).end()
#define mp make_pair
#define pb push_back
#define sz(x) (int)(x).size()
#define INF 1000000000
#define M 1000000007ll
ll h,w,a,b,f[300005],inv[300005];
ll expo(ll a,ll b) {
	if (!b) return 1ll;
	ll x=expo(a,b/2ll);
	if (b%2ll) return (((x*x)%M)*a)%M;
	return (x*x)%M;
}
ll choose(int n,int k) {
	return (((f[n]*inv[n-k])%M)*inv[k])%M;
}
int main() {
	scanf("%lld%lld%lld%lld",&h,&w,&a,&b);
	ll n=(h+w-2);
	f[0]=inv[0]=1ll;
	for (int i=1;i<=n;i++) {
		f[i]=(f[i-1]*(ll)i)%M;
		inv[i]=expo(f[i],M-2ll);
	}
	ll ans=0ll;
	for (ll l=h-a-1+b;l<=n-a;l++) {
		ans+=choose(l,h-a-1)*choose(n-l-1,a-1);
		ans%=M;
	}
	printf("%lld\n", ans);
}