// Slower C++ solution:
#include <bits/stdc++.h>

 

using namespace std;



#define rep(i, a) for (int i = 0; i < (int) a; i++)

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



const int mod = 1000000007;



using ll = long long;



vector<ll> fact(300000);



void init()

{

	fact[0] = 1;

	for (int i = 1; i < 300000; i++) {

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

		fact[i] %= mod;

	}

}



ll modpow(ll val, int p = mod - 2)

{

	if (p == 0) return 1;

	ll res = modpow(val, p / 2);

	res = res * res % mod;

	if (p % 2 == 1) return res * val % mod;

	return res;

}



ll comb(int n, int r)

{

	return (fact[n] * modpow(fact[n-r]) % mod) * modpow(fact[r]) % mod;

}



signed main()

{

	init();

	ll res = 0;

	int H, W, A, B;

	cin >> H >> W >> A >> B;

	int h, w;

	for (h = H - A - 1, w = B; h >= 0 && w < W; h--, w++) {

		ll t = comb(h+w, h);

		ll a, b;

		a = H-h-1;

		b = W-w-1;

		res += t * comb(a+b, a) % mod;

		res %= mod;

	}

	cout << res << endl;

}


// Faster C++ solution for the same problem (measured speedup: 4.97974600788072x):
#include<bits/stdc++.h>



using namespace std;



namespace mylib {

template<int mod>

struct ModInt {

	using ll = long long;

	int x;

	const static int M = mod;

	ModInt(ll x_) {

		x = x_ % mod;

		if (x < 0) x += mod;

	}

	ModInt() : ModInt(0) {}

	inline ModInt& operator+=(const ModInt rhs) {

		x += rhs.x;

		x %= mod;

		return (*this);

	}

	inline ModInt& operator-=(const ModInt rhs) {

		x += -rhs;

		return *this;

	}

	inline ModInt& operator*=(const ModInt rhs) {

		x = ((ll) x * rhs.x) % mod;

		return *this;

	}

	inline ModInt& operator/=(ModInt rhs) {

		return *this *= rhs.power(mod - 2);

	}

	inline ModInt power(ll p) {

		ModInt res = 1;

		ModInt a = x;

		for (; p; res = p & 1 ? res * a : res, a *= a, p >>= 1);

		return res;

	}

	inline ModInt& operator=(const ModInt& rhs) {

		this->x = rhs.x;

		return *this;

	}

	inline int operator==(const ModInt& rhs) const {

		return this->x == rhs.x;

	}

	inline int operator!=(const ModInt& rhs) const {

		return !(*this != rhs);

	}

	inline ModInt operator++(signed unused) {

		ModInt res(*this);

		++(*this);

		return res;

	}

	inline ModInt& operator++() {

		(*this) += 1;

		return (*this);

	}

	inline ModInt operator--(signed unused) {

		ModInt res(*this);

		--(*this);

		return res;

	}

	inline ModInt& operator--() {

		(*this) -= 1;

		return (*this);

	}

	inline ModInt operator+() const {

		return (*this);

	}

	inline ModInt operator-() const {

		return (*this).x ? ModInt(mod - (*this).x) : ModInt(0);

	}

	friend const ModInt operator+(const ModInt& lhs, const ModInt& rhs) {return ModInt(lhs) += rhs;}

	friend const ModInt operator-(const ModInt& lhs, const ModInt& rhs) {return ModInt(lhs) -= rhs;}

	friend const ModInt operator*(const ModInt& lhs, const ModInt& rhs) {return ModInt(lhs) *= rhs;}

	friend const ModInt operator/(const ModInt& lhs, const ModInt& rhs) {return ModInt(lhs) /= rhs;}

	explicit operator int() const {return x;}

	friend ostream& operator<<(ostream& lhs, const ModInt& rhs) {

		lhs << rhs.x;

		return lhs;

	}

	friend istream& operator>>(istream& lhs, ModInt& rhs) {

		long long t;

		lhs >> t;

		rhs = ModInt(t);

		return lhs;

	}

};

using modint = ModInt<1000000007>;

//using modint = ModInt<998244353>;

modint operator"" _mi(unsigned long long lit) {

	return modint(lit % modint::M);

}

}; // mylib

using namespace mylib;



template<class T>

struct ModComb {

	vector<T> fact;

	vector<T> facti;

	ModComb(int n) : fact(n+1), facti(n+1) {

		fact[0] = 1;

		for (int i = 1; i < n + 1; i++) {

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

		}

		facti[n] = 1 / fact[n];

		for (int i = n-1; i >= 0; i--) {

			facti[i] = facti[i+1] * (i + 1);

		}

	}

	T C(int n, int r) const {

		if(n < 0 || r < 0 || n < r) return 0;

		return fact[n] * facti[r] * facti[n-r];

	}

	T P(int n, int r) const {

		if(n < 0 || r < 0 || n < r) return 0;

		return fact[n] * facti[n-r];

	}

	T H(int n, int r) const {

		if(n < 0 || r < 0) return 0;

		return C(n + r - 1, r);

	}

};



using modcomb = ModComb<modint>;



int main() {

	modint res = 0;

	int H, W, A, B;

	cin >> H >> W >> A >> B;

	modcomb m(W + H);

	int r = H - A - 1;

	int c = B;

	for (; r >= 0 && c < W; r--, c++) {

		int C = W - c - 1;

		int R = H - r - 1;

		res += m.C(r + c, r) * m.C(R + C, R);

	}

	cout << res << endl;

}