// tested by Hightail - https://github.com/dj3500/hightail
import std.stdio, std.string, std.conv, std.algorithm;
import std.range, std.array, std.math, std.typecons, std.container, core.bitop;

int w, h, a, b;

void main() {
    scan(h, w, a, b);

    auto c = ModCombo(w + h);

    long ans;

    foreach (i ; 0 .. h - a) {
        (ans += c.binom(b + i - 1, i) * c.binom(w + h - b - i - 2, h - i - 1) % c.mod) %= c.mod;
    }

    writeln(ans);
}


struct ModCombo {
    private {
        immutable long _mod;
        long[] _f;
        long[] _finv;

        long _powmod(long x, long y, long mod) {
            return y > 0 ? _powmod(x, y>>1, mod)^^2 % mod * x^^(y&1) % mod : 1L;
        }
    }

    @property {
        long mod() {return _mod;}
        long f(int n) {return _f[n];}
        long finv(int n) {return _finv[n];}
    }

    this (int N, long mod = 10^^9 + 7) {
        _mod = mod;
        _f = new long[](N + 1);
        _finv = new long[](N + 1);
        _f[0] = _finv[0] = 1L;

        foreach (i ; 1 .. N + 1) {
            _f[i] = i * _f[i - 1] % _mod;
        }

        _finv[N] = _powmod(_f[N], _mod - 2, _mod);

        foreach_reverse (i ; 1 .. N) {
            _finv[i] = (i + 1) * _finv[i + 1] % _mod;
        }
    }

    long perm(int n, int k) {
        return _f[n] * _finv[k] % _mod;
    }

    long binom(int n, int k) {
        return _f[n] * _finv[k] % _mod * _finv[n - k] % _mod;
    }

    long H(int n, int k) {
        if (n == 0 && k == 0) return 1L;
        return _f[n + k - 1] * _finv[k] % _mod * _finv[n - 1] % _mod;
    }
}


void scan(T...)(ref T args) {
    string[] line = readln.split;
    foreach (ref arg; args) {
        arg = line.front.to!(typeof(arg));
        line.popFront();
    }
    assert(line.empty);
}


void fillAll(R, T)(ref R arr, T value) {
    static if (is(typeof(arr[] = value))) {
        arr[] = value;
    }
    else {
        foreach (ref e; arr) {
            fillAll(e, value);
        }
    }
}