//
//  main.swift
//  Atcoder
//
//  Created by Louis Joe Shen on 2020/7/17.
//  Copyright © 2020 Louise Joe Shen. All rights reserved.
//

import Foundation

var input = readLine()!.split(separator: " ")

var H = Int(input[0])!
var W = Int(input[1])!
var A = Int(input[2])!
var B = Int(input[3])!

var MOD : Int64 = 1000000000 + 7

func C(r : Int,k : Int) -> Int64{
    if k == 0{
        return 1
    }
    var alpha : Int64 = 1
    var beta : Int64 = 1
    var delta : Int64 = 0
    
    for i in r - k + 1 ... r {
        alpha %= MOD
        delta = Int64(i)
        delta %= MOD
        alpha *= delta
        alpha %= MOD
    }
    
    for i in 1 ... k {
        beta %= MOD
        delta = Int64(i)
        delta %= MOD
        beta *= delta
        beta %= MOD
    }
    
    return alpha / beta % MOD
}

var n = H - A
var ans : Int64 = 0
for i in 1 ... n{
    ans += ((C(r: B + i - 2, k: i - 1) % MOD) * (C(r: W + H - B - i - 1, k: H - i) % MOD)) % MOD
    ans %= MOD
}

print(ans)
