#include "bits/stdc++.h"

#define REP(i,n) for(ll i=0;i<ll(n);++i)
#define RREP(i,n) for(ll i=ll(n)-1;i>=0;--i)
#define FOR(i,m,n) for(ll i=m;i<ll(n);++i)
#define RFOR(i,m,n) for(ll i=ll(n)-1;i>=ll(m);--i)
#define ALL(v) (v).begin(),(v).end()
#define UNIQUE(v) v.erase(unique(ALL(v)),v.end());
#define INF 1000000001ll
#define MOD 1000000007ll
#define EPS 1e-9

constexpr int dx[8] = { 1,1,0,-1,-1,-1,0,1 };
constexpr int dy[8] = { 0,1,1,1,0,-1,-1,-1 };


using namespace std;

using ll = long long;
using vi = vector<int>;
using vl = vector<ll>;
using vvi = vector<vi>;
using vvl = vector<vl>;
using pii = pair<int, int>;
using pll = pair<ll, ll>;

template <class T> bool chmin(T &a, T b) { if (a > b) { a = b; return true; } return false; }
template <class T> bool chmax(T &a, T b) { if (a < b) { a = b; return true; } return false; }
#include "bits/stdc++.h"
template<int mod, bool isPrime = false >
class  Mint {
	using ll = long long;
	ll value;
public:
	constexpr Mint() : value(0) {};
	constexpr Mint(ll x) : value(x) {
		value %= mod;
		if (value < 0)value += mod;
	}
	constexpr Mint(const Mint &other) : value(other.value) {}

	ll get() const { return value; }

	constexpr Mint pow(ll n) {
		Mint ret(1), tmp(value);
		while (n) {
			if (n & 1)ret *= tmp;
			n >>= 1;
			tmp *= tmp;
		}
		return ret;
	}
	constexpr bool operator == (const Mint &a) const {
		return value == a.value;
	}
	constexpr Mint& operator =(const Mint &a) {
		value = a.value;
		return *this;
	}
	constexpr Mint& operator +=(const Mint &a) {
		if ((value += a.value) >= mod)value -= mod;
		return *this;
	}
	constexpr Mint& operator -=(const Mint &a) {
		if ((value += mod - a.value) >= mod)value -= mod;
		return *this;
	}
	constexpr Mint& operator *=(const Mint &a) {
		value = value * a.value% mod;
		return *this;
	}

	constexpr Mint& operator /=(const Mint &a) {
		return *this *= inv(a);
	}
	constexpr Mint operator +(const Mint &a) const {
		return Mint(*this) += a;
	}
	constexpr Mint operator -(const Mint &a) const {
		return Mint(*this) -= a;
	}
	constexpr Mint operator *(const Mint &a) const {
		return Mint(*this) *= a;
	}
	constexpr Mint operator /(const Mint &a) const {
		return Mint(*this) /= a;
	}
};
template<int mod, bool isPrime> std::ostream& operator << (std::ostream &os, const Mint<mod, isPrime>& m) { os << m.get(); return os; }
template<int mod, bool isPrime> std::istream& operator >> (std::istream &is, Mint<mod, isPrime>& m) { is >> m.get();  return is; }

template<int mod>
constexpr Mint<mod, true> inv(const Mint<mod, true> &m) {
	long long a = m.get(), b = mod, u = 1, v = 0;
	while (b) {
		long long t = a / b;
		a -= t * b; std::swap(a, b);
		u -= t * v; std::swap(u, v);
	}
	return Mint<mod, true>(u);
}

template<int mod, bool isPrime>
constexpr Mint<mod, isPrime> operator +(long long lhs, const Mint<mod, isPrime> &a) {
	return Mint<mod, isPrime>(lhs + a.get());
}

template<int mod, bool isPrime>
constexpr Mint<mod, isPrime> operator -(long long lhs, const Mint<mod, isPrime> &a) {
	return Mint<mod, isPrime>(lhs - a.get());
}

template<int mod, bool isPrime>
constexpr Mint<mod, isPrime> operator *(long long lhs, const Mint<mod, isPrime> &a) {
	return Mint<mod, isPrime>(lhs%mod * a.get());
}

constexpr int default_mod = 1000000007;
using mint = Mint<default_mod, true>;

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);
	int h, w, a, b;
	cin >> h >> w >> a >> b;
	mint ans = 0;
	vector<mint> fact(h + w), rfact(h + w);
	fact[0] = 1;
	REP(i, fact.size() - 1)fact[i + 1] = fact[i] * (i + 1);
	rfact[h + w - 1] = inv(fact.back());
	RREP(i, rfact.size() - 1)rfact[i] = rfact[i + 1] * (i + 1);
	auto comb = [&](int r, int c) {
		return fact[r] * rfact[c] * rfact[r - c];
	};
	ans += comb(h + w - 2, h - 1);
	REP(i, a) {
		ans -= comb(h - i + b - 2, h - i - 1)*comb(i + 1 + w - b - 2, i);
	}
	cout << ans << endl;
}

Measured memory_bytes: 3328