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
    
    for i in r - k + 1 ... r {
        alpha *= Int64(i)
        alpha %= MOD
    }
    
    for i in 1 ... k {
        beta *= Int64(i)
        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) * C(r: W + H - B - i - 1, k: H - i)
    ans %= MOD
}

print(ans)