// need
#include <iostream>
#include <algorithm>

// data structure
#include <bitset>
//#include <list>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <utility>
#include <vector>
//#include <array>
//#include <tuple>
//#include <unordered_map>
#include <unordered_set>
#include <complex>
//#include <deque>
#include<valarray>

// stream
//#include <istream>
//#include <sstream>
//#include <ostream>
#include <fstream>

// etc
#include <cmath>
#include <cassert>
#include <functional>
#include <iomanip>
//#include <typeinfo>
#include <chrono>
#include <random>
#include <numeric>


#define INIT std::ios::sync_with_stdio(false);std::cin.tie(0);
#define VAR(type, ...)type __VA_ARGS__;MACRO_VAR_Scan(__VA_ARGS__);
template<typename T> void MACRO_VAR_Scan(T& t) { std::cin >> t; }
template<typename First, typename...Rest>void MACRO_VAR_Scan(First& first, Rest&...rest) { std::cin >> first; MACRO_VAR_Scan(rest...); }
#define VEC_ROW(type, n, ...)std::vector<type> __VA_ARGS__;MACRO_VEC_ROW_Init(n, __VA_ARGS__); for(int i=0; i<n; ++i){MACRO_VEC_ROW_Scan(i, __VA_ARGS__);}
template<typename T> void MACRO_VEC_ROW_Init(int n, T& t) { t.resize(n); }
template<typename First, typename...Rest>void MACRO_VEC_ROW_Init(int n, First& first, Rest&...rest) { first.resize(n); MACRO_VEC_ROW_Init(n, rest...); }
template<typename T> void MACRO_VEC_ROW_Scan(int p, T& t) { std::cin >> t[p]; }
template<typename First, typename...Rest>void MACRO_VEC_ROW_Scan(int p, First& first, Rest&...rest) { std::cin >> first[p]; MACRO_VEC_ROW_Scan(p, rest...); }
#define OUT(d) std::cout<<d;
#define FOUT(n, d) std::cout<<std::fixed<<std::setprecision(n)<<d;
#define SOUT(n, c, d) std::cout<<std::setw(n)<<std::setfill(c)<<d;
#define SP std::cout<<" ";
#define TAB std::cout<<"\t";
#define BR std::cout<<"\n";
#define SPBR(i, n) std::cout<<(i + 1 == n ? '\n' : ' ');
#define ENDL std::cout<<std::endl;
#define FLUSH std::cout<<std::flush;
#define VEC(type, c, n) std::vector<type> c(n);for(auto& i:c)std::cin>>i;
#define MAT(type, c, m, n) std::vector<std::vector<type>> c(m, std::vector<type>(n));for(auto& r:c)for(auto& i:r)std::cin>>i;
#define ALL(a) (a).begin(),(a).end()
#define FOR(i, a, b) for(int i=(a);i<(b);++i)
#define RFOR(i, a, b) for(int i=(b)-1;i>=(a);--i)
#define REP(i, n) for(int i=0;i<int(n);++i)
#define RREP(i, n) for(int i=(n)-1;i>=0;--i)
#define FORLL(i, a, b) for(ll i=ll(a);i<ll(b);++i)
#define RFORLL(i, a, b) for(ll i=ll(b)-1;i>=ll(a);--i)
#define REPLL(i, n) for(ll i=0;i<ll(n);++i)
#define RREPLL(i, n) for(ll i=ll(n)-1;i>=0;--i)
#define IN(a, x, b) (a<=x && x<b)
#define SHOW(d) {std::cerr << #d << "\t:" << d << "\n";}
#define SHOWVECTOR(v) {std::cerr << #v << "\t:";for(const auto& xxx : v){std::cerr << xxx << " ";}std::cerr << "\n";}
#define SHOWVECTOR2(v) {std::cerr << #v << "\t:\n";for(const auto& xxx : v){for(const auto& yyy : xxx){std::cerr << yyy << " ";}std::cerr << "\n";}}
#define SHOWQUEUE(a) {auto tmp(a);std::cerr << #a << "\t:";while(!tmp.empty()){std::cerr << tmp.front() << " ";tmp.pop();}std::cerr << "\n";}
template<typename T> inline T CHMAX(T& a, const T b) { return a = (a < b) ? b : a; }
template<typename T> inline T CHMIN(T& a, const T b) { return a = (a > b) ? b : a; }
#define EXCEPTION(msg) throw std::string("Exception : " msg " [ in ") + __func__ + " : " + std::to_string(__LINE__) + " lines ]"
#define TRY(cond, msg) try {if (cond) EXCEPTION(msg);}catch (std::string s) {std::cerr << s << std::endl;}
void CHECKTIME(std::function<void()> f) { auto start = std::chrono::system_clock::now(); f(); auto end = std::chrono::system_clock::now(); auto res = std::chrono::duration_cast<std::chrono::nanoseconds>((end - start)).count(); std::cerr << "[Time:" << res << "ns  (" << res / (1.0e9) << "s)]\n"; }

template<typename S, typename T>
std::ostream& operator<<(std::ostream& os, std::pair<S, T> p) {
	os << "(" << p.first << ", " << p.second << ")";
	return os;
}

#define int ll
using ll = long long;
using ull = unsigned long long;
using PAIR = std::pair<int, int>;
using PAIRLL = std::pair<ll, ll>;
constexpr int INFINT = 1 << 30;                          // 1.07x10^ 9
constexpr int INFINT_LIM = (1LL << 31) - 1;              // 2.15x10^ 9
constexpr ll INFLL = 1LL << 60;                          // 1.15x10^18
constexpr ll INFLL_LIM = (1LL << 62) - 1 + (1LL << 62);  // 9.22x10^18
constexpr double EPS = 1e-7;
constexpr int MOD = 1000000007;
constexpr double PI = 3.141592653589793238462643383279;

//組み合わせ(コンビネーション)
const int FACT_MAX_N = 1000000;
int fact[FACT_MAX_N];
void FactrialInitialize(ll n, ll mod) {
	fact[0] = 1;
	FORLL(i, 1, n) {
		fact[i] = i * fact[i - 1] % mod;
	}
}
ll Factrial(ll n, ll mod) {
	if (fact[0] == 0) FactrialInitialize(FACT_MAX_N, mod);
	if (fact[n] != 0) return fact[n];
	if (n == 0) return fact[n] = 1;
	return fact[n] = Factrial(n - 1, mod) * n % mod;
}

ll powMod(ll n, ll p, ll mod) {
	if (n == 0) return 0;
	if (p <= 0 || n == 1) return 1;
	if (p % 2 == 0) {
		ll t = powMod(n, p / 2, mod);
		return t*t % mod;
	}
	return n*powMod(n, p - 1, mod) % mod;
}

ll Permutation(ll n, ll r, ll mod) {
	if (n < r) return 0;
	if (n == r) return Factrial(n, mod);
	ll ans = 1;
	ans *= Factrial(n, mod); ans %= mod;
	ans *= powMod(Factrial(n - r, mod), mod - 2, mod); ans %= mod;
	return ans;
}

/*fact準備O(n), クエリO(log(mod))*/
ll Combination(ll n, ll r, ll mod) {
	if (r == 0) return 1;
	if (n < r) return 0;
	ll ans = 1;
	ans *= Factrial(n, mod); ans %= mod;
	ans *= powMod(Factrial(n - r, mod), mod - 2, mod); ans %= mod;
	ans *= powMod(Factrial(r, mod), mod - 2, mod); ans %= mod;
	return ans;
}
/*n<=10^9, r<=10^5のときなど, O(r)*/
/*ll Combination(ll n, ll r, ll mod) {
	if (n < r) return 0;
	ll ans = 1;
	if (n < 2 * r) r = n - r;
	for (int i = 1; i <= r; ++i) {
		ans *= n - i + 1; ans %= mod;
		ans *= powMod(i, mod - 2, mod); ans %= mod;
	}
	return ans;
}*/

ll CombinationWithRepetition(ll n, ll r, ll mod) {
	return Combination(n + r - 1, r, mod);
}

signed main() {
	INIT;
	VAR(int, h, w, a, b);
	int res = Combination(h + w - 2, h - 1, MOD);
	FOR(i, 1, a + 1) {
		res -= Combination(h - a + b + i - 2, b - 1, MOD) * Combination(a - i + 1 + w - b - 2, w - b - 1, MOD) % MOD;
		res %= MOD;
		res += MOD;
		res %= MOD;
	}
	OUT(res)BR;
	return 0;
}

Measured memory_bytes: 8064