#include <bits/stdc++.h>
using namespace std;

int H, W, A, B;

template<int MOD = 1000000007>
struct ModInt {
    int val;
    ModInt(long long val_ = 0) : val(val_ >= 0 ? val_ % MOD : (MOD - (-val_) % MOD) % MOD) {}
    bool operator==(const ModInt &rhs) const { return val == rhs.val; }
    bool operator!=(const ModInt &rhs) const { return val != rhs.val; }
    ModInt &operator+=(const ModInt &rhs) {
        if ((val += rhs.val) >= MOD) { val -= MOD; }
        return *this;
    }
    ModInt &operator-=(const ModInt &rhs) {
        if ((val += MOD - rhs.val) >= MOD) { val -= MOD; }
        return *this;
    }
    ModInt &operator*=(const ModInt &rhs) {
        val = (int) ((long long) val * rhs.val % MOD);
        return *this;
    }
    ModInt &operator/=(const ModInt &rhs) { return *this *= rhs.inv(); }
    ModInt operator+() const { return *this; }
    ModInt operator-() const { return ModInt(-val); }
    ModInt operator++() { return *this += 1; }
    ModInt operator--() { return *this -= 1; }
    ModInt operator++(signed) {
        const ModInt ret(*this);
        ++*this;
        return ret;
    }
    ModInt operator--(signed) {
        const ModInt ret(*this);
        --*this;
        return ret;
    }
    ModInt operator+(const ModInt &rhs) const { return ModInt(*this) += rhs; }
    ModInt operator-(const ModInt &rhs) const { return ModInt(*this) -= rhs; }
    ModInt operator*(const ModInt &rhs) const { return ModInt(*this) *= rhs; }
    ModInt operator/(const ModInt &rhs) const { return ModInt(*this) /= rhs; }
    ModInt inv() const {
        int a = MOD, b = val, u = 0, v = 1;
        while (b > 0) {
            int t = a / b;
            std::swap(a -= t * b, b);
            std::swap(u -= t * v, v);
        }
        return ModInt(u);
    }
    ModInt pow(long long n) const {
        ModInt ret = 1, mul = *this;
        if (n < 0) { n = -n, mul = mul.inv(); }
        while (n > 0) {
            if (n & 1) { ret *= mul; }
            mul *= mul;
            n >>= 1;
        }
        return ret;
    }
    friend std::istream &operator>>(std::istream &is, ModInt &rhs) {
        long long v;
        is >> v;
        rhs = v;
        return is;
    }
    friend std::ostream &operator<<(std::ostream &os, const ModInt &rhs) { return os << rhs.val; }
    struct Combination {
        std::vector<ModInt> fact{1, 1}, fInv{1, 1}, inv{0, 1};
        void calc(int n) {
            while (fact.size() <= n) {
                int i = fact.size();
                fact.push_back(fact[i - 1] * i);
                inv.push_back(-inv[MOD % i] * (MOD / i));
                fInv.push_back(fInv[i - 1] * inv[i]);
            }
        }
        ModInt P(int n, int r) { return r < 0 || n < r ? 0 : (calc(n), fact[n] * fInv[n - r]); }
        ModInt C(int n, int r) { return r < 0 || n < r ? 0 : (calc(n), fact[n] * fInv[r] * fInv[n - r]); }
        ModInt H(int n, int r) { return C(n + r - 1, r); }
    };
};

signed main() {
    cin >> H >> W >> A >> B;
    using Mint=ModInt<>;
  	Mint::Combination Comb;
  	Mint ans = 0;
    for (int i = 1; i <= H - A; i++) {
        ans += Comb.C(i + B - 2, B - 1) * Comb.C(H + W - B - i - 1, W - B - 1);
    }
    cout << ans << endl;
    return 0;
}


Measured memory_bytes: 2784