import std.stdio, std.string, std.conv;
import std.range, std.algorithm, std.array, std.typecons, std.container;
import std.math, std.numeric, core.bitop;

enum inf = 1<<30;
enum mod = 10L^^9 + 7;

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

    auto fact = new long[](h + w + 1);
    auto rfact = new long[](h + w + 1);
    fact[0] = 1;
    foreach (i ; 1 .. h + w + 1) {
        fact[i] = (i * fact[i-1]) % mod;
    }
    rfact[h + w] = powmod(fact[h + w], mod - 2);
    foreach_reverse (i ; 0 .. h + w) {
        rfact[i] = (rfact[i+1] * (i + 1)) % mod;
    }

    long comb(int n, int r) {
        return fact[n] * rfact[n-r] % mod * rfact[r] % mod;
    }

    long ans;

    foreach (i ; 0 .. w - b) {
        ans += comb(h - a - 1 + b + i, h - a - 1) * comb(a - 1 + w - b - 1 - i, a - 1) % mod;
        if (ans > mod) ans-= mod;
    }

    writeln(ans);
}



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












































void scan(T...)(ref T args) {
    import std.stdio : readln;
    import std.algorithm : splitter;
    import std.conv : to;
    import std.range.primitives;

    auto line = readln().splitter();
    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);
        }
    }
}
