// Original C++ solution (slower):
#include<bits/stdc++.h>

using namespace std;

//#include<ext/rope>

//using namespace __gnu_cxx

//#include<ext/pb_ds/priority_queue.hpp>

//using namespace __gnu_pbds;

#define lowbit(x) (x&-x)

#define pb push_back

#define empb emplace_back

#define all(x) (x).begin(),(x).end()

#define clr(a,b) memset(a,b,sizeof(a))

#define caze(T) for(cin>>T;T;T--)

#define inf (1<<29)

#define Endl ('\n')

#define fi first

#define se second

#define db double

#define ldb long double

#define ll long long

#define pii pair<int,int>

#define pil pair<int,ll>

#define pli pair<ll,int>

#define pll pair<ll,ll>

#define ull unsigned long long

#define IOS ios::sync_with_stdio(0),cin.tie(0),cout.tie(0)

inline char nc() {

	#define SZ 1000000

	static char buf[SZ], *p1, *p2;

	return p1==p2&&(p2=(p1=buf)+fread(buf,1,SZ,stdin),p1==p2)?EOF:*p1++;

}

template<typename T>

inline int scan(T &x) {

	char c, sign=1; 

	while(!isdigit(c=nc())&&c!=EOF)if(c=='-')sign*=-1;

	if(c==EOF) return EOF;

	for(x = 0; isdigit(c); c = nc())

		x = (x<<1) + (x<<3) + (c&15);

	return x *= sign, 1;

}

template<typename T, typename...Args>

inline int scan(T &x, Args&...args) {

	return scan(x), scan(args...);

}

const int N=1<<18;

const int mod=1000000007;

int f[N],ivf[N];

int qp(int a,int b)

{

	int ret=1;

	for(;b;b>>=1,a=1LL*a*a%mod)

		if(b&1) ret=1LL*ret*a%mod;

	return ret;

}

int C(int n,int m){int ret=1LL*f[n]*ivf[n-m]%mod;return 1LL*ret*ivf[m]%mod;}

int go(int n,int m){return C(n+m-2,m-1);}

int main()

{

	f[0]=1;

	for(int i=1;i<N;++i)

		f[i]=1LL*f[i-1]*i%mod;

	ivf[N-1]=qp(f[N-1],mod-2);

	for(int i=N-2;~i;--i)

		ivf[i]=1LL*ivf[i+1]*(i+1)%mod;

	int n,m,a,b;

	while(cin>>n>>m>>a>>b)

	{

		int ans=0;

		for(int i=1;i<=n-a;++i)

		{

			int tp=1LL*go(i,b)*go(n-i+1,m-b)%mod;

			(ans+=tp)%=mod;

		}

		cout<<ans<<Endl;

	}

}

// Optimized solution to the same problem (faster):
// #pragma GCC optimize("Ofast,O3,unroll-loops")

// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,fma")



#include <vector>

#include <list>

#include <map>

#include <set>

#include <deque>

#include <queue>

#include <stack>

#include <bitset>

#include <algorithm>

#include <functional>

#include <numeric>

#include <utility>

#include <iostream>

#include <sstream>

#include <iomanip>

#include <cstdio>

#include <cmath>

#include <cstdlib>

#include <cctype>

#include <string>

#include <cstring>

#include <cstdio>

#include <cmath>

#include <cstdlib>

#include <ctime>

#include <climits>

#include <complex>

#include <ext/pb_ds/assoc_container.hpp>

#include <ext/pb_ds/tree_policy.hpp>



using namespace __gnu_pbds;

using namespace std;



#define endl '\n'

#define hoise cerr<<"hoise - "<<__LINE__<<"\n"

#define tham getchar()

#define set0(ar) memset(ar, 0, sizeof ar)

#define setinf(ar) memset(ar, 126, sizeof ar)

#define vsort(v) sort(v.begin(), v.end())

#define all(x) (x).begin(), (x).end()

#define FASTIO ios::sync_with_stdio(false);cin.tie(nullptr);



using ll = long long;

using pii = pair <int, int>;

using pll = pair <ll, ll>;

using vi = vector <int>;

using vl = vector <ll>;

using vpii = vector < pii >;

using vpll = vector < pll >;



const int MXN = (int)2e5+6;

const double PI = acos(-1.0);

const ll MOD = 1e9 + 7;

const int INF = 1e9;

const double EPS = 1e-6;

const int dx[4] = {1,0,-1,0}, dy[4] = {0,1,0,-1};



#define error(args...) { string _s = #args; replace(_s.begin(), _s.end(), ',', ' '); stringstream _ss(_s); istream_iterator<string> _it(_ss); err(_it, args); }

void err(istream_iterator<string> it) {}

template<typename T, typename... Args>

void err(istream_iterator<string> it, T a, Args... args) {

    cerr << *it << " = " << a << endl;

    err(++it, args...);

}



int flipBit (int mask, int k) { return mask = mask ^ (1 << k); }

bool checkBit (int mask, int k) { return (mask >> k) & 1; }

int setBit (int mask, int k) { return mask = mask | (1 << k); }

int resetBit (int mask, int k) { if (checkBit (mask, k)) { mask = flipBit (mask, k); } return mask; }



inline void normal(ll &a) { a %= MOD; (a < 0) && (a += MOD); }

inline ll modMul(ll a, ll b) { a %= MOD, b %= MOD; normal(a), normal(b); return (a * b) % MOD; }

inline ll modAdd(ll a, ll b) { a %= MOD, b %= MOD; normal(a), normal(b); return (a + b) % MOD; }

inline ll modSub(ll a, ll b) { a %= MOD, b %= MOD; normal(a), normal(b); a -= b; normal(a); return a; }

inline ll modPow(ll b, ll p) { ll r = 1; while (p) { if (p & 1) r = modMul(r, b); b = modMul(b, b); p >>= 1; } return r; }

inline ll modInverse(ll a) { return modPow(a, MOD - 2); }

inline ll modDiv(ll a, ll b) { return modMul(a, modInverse(b)); }



template <typename T> using orderedSet = tree<T, null_type, greater<T>, rb_tree_tag, tree_order_statistics_node_update>;

template <typename T> using orderedSet1 = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;



bool approximatelyEqual(float a, float b, float epsilon) {

    return fabs(a - b) <= ( (fabs(a) < fabs(b) ? fabs(b) : fabs(a)) * epsilon);

}

bool essentiallyEqual(float a, float b, float epsilon) {

    return fabs(a - b) <= ( (fabs(a) > fabs(b) ? fabs(b) : fabs(a)) * epsilon);

}

bool definitelyGreaterThan(float a, float b, float epsilon) {

    return (a - b) > ( (fabs(a) < fabs(b) ? fabs(b) : fabs(a)) * epsilon);

}

bool definitelyLessThan(float a, float b, float epsilon) {

    return (b - a) > ( (fabs(a) < fabs(b) ? fabs(b) : fabs(a)) * epsilon);

}



/*--------------------------------------------------------------------------------------------------------------------------*/



ll fact[MXN], inv[MXN];



ll ncr (ll n, ll r) {

	return (fact[n] % MOD * inv[n - r] % MOD * inv[r] % MOD) % MOD;

}



void solve () {

	

	ll h, w, a, b;

	cin >> h >> w >> a >> b;

	

	fact[0] = 1;

	for (int i = 1; i <= h + w; ++i) {

		fact[i] = fact[i - 1] * i;

		fact[i] %= MOD;

	}

	

	for (int i = 0; i <= h + w; ++i) {

		inv[i] = modInverse (fact[i]);

		inv[i] %= MOD;

	}

	

	ll ans = 0;

	

	for (int i = b; i < w; ++i) {

		ll comb1 = ncr (h + i - a - 1, i);

		ll comb2 = ncr (w + a - i - 2, a - 1);

		ans += comb1 * comb2;

		ans %= MOD;

	}

	cout << ans << endl;

}



int main(){

	// FASTIO

	

	int t;

	// cin >> t;

	t = 1;

	

	while (t--) {

		solve ();

	}

	

    return 0;

}



/*

 * be careful before submitting in Lightoj, SPOJ, UVA

*/


// Fastest known solution for this problem: