# ARC058D - いろはちゃんとマス目 / Iroha and a Grid (ABC042D)

def get_fact(lim):

    # compute a toble of factorials (1-idx)

    fact = [1] * (lim + 1)

    x = 1

    for i in range(1, lim + 1):

        x = (x * i) % MOD

        fact[i] = x

    return fact



def get_inv(lim):

    # compute a toble of inverse factorials (1-idx)

    inv = [1] * (lim + 1)

    inv[lim] = pow(fact[lim], MOD - 2, MOD)

    x = inv[lim]

    for i in range(lim - 1, 0, -1):

        x = (x * (i + 1)) % MOD

        inv[i] = x

    return inv



def comb(n: int, r: int) -> int:

    # compute nCr (n! / r!(n - r)!)

    return fact[n] * inv[n - r] * inv[r]





def main():

    global MOD, fact, inv

    H, W, A, B = tuple(map(int, input().split()))

    MOD = 10 ** 9 + 7

    fact = get_fact(H + W)

    inv = get_inv(H + W)

    ans = 0

    for i in range(B, W):

        ans += comb(i + H - A - 1, i) * comb(W - i - 1 + A - 1, A - 1)

        ans %= MOD

    print(ans)





if __name__ == "__main__":

    main()
# ARC058D - いろはちゃんとマス目 / Iroha and a Grid (ABC042D)

def get_fact(lim):

    # compute a toble of factorials (1-idx)

    fact = [1] * (lim + 1)

    x = 1

    for i in range(1, lim + 1):

        x = (x * i) % MOD

        fact[i] = x

    return fact



def get_inv(lim):

    # compute a toble of inverse factorials (1-idx)

    inv = [1] * (lim + 1)

    inv[lim] = pow(fact[lim], MOD - 2, MOD)

    x = inv[lim]

    for i in range(lim - 1, 0, -1):

        x = (x * (i + 1)) % MOD

        inv[i] = x

    return inv



def comb(n: int, r: int) -> int:

    # compute nCr (n! / r!(n - r)!)

    return fact[n] * inv[n - r] * inv[r]





def main():

    global MOD, fact, inv

    H, W, A, B = tuple(map(int, input().split()))

    MOD = 10 ** 9 + 7

    fact = get_fact(H + W)

    inv = get_inv(H + W)

    ans = 0

    x, y = H - A - 1, W + A - 2

    for i in range(B, W):

        ans += comb(x + i, i) * comb(y - i, A - 1)

    ans %= MOD

    print(ans)





if __name__ == "__main__":

    main()
# ARC058D - いろはちゃんとマス目 / Iroha and a Grid (ABC042D)
def get_fact(lim):
    # compute a toble of factorials (1-idx)
    fact = [1] * (lim + 1)
    x = 1
    for i in range(1, lim + 1):
        x = (x * i) % MOD
        fact[i] = x
    return fact


def get_inv(lim):
    # compute a toble of inverse factorials (1-idx)
    inv = [1] * (lim + 1)
    inv[lim] = pow(fact[lim], MOD - 2, MOD)
    x = inv[lim]
    for i in range(lim - 1, 0, -1):
        x = (x * (i + 1)) % MOD
        inv[i] = x
    return inv


def comb(n: int, r: int) -> int:
    # compute nCr (n! / r!(n - r)!)
    return fact[n] * inv[n - r] * inv[r]


def main():
    global MOD, fact, inv
    H, W, A, B = tuple(map(int, input().split()))
    MOD = 10**9 + 7
    fact = get_fact(H + W)
    inv = get_inv(H + W)
    ans = 0
    for i in range(B, W):
        ans += comb(i + H - A - 1, i) * comb(W - i - 1 + A - 1, A - 1)
        ans %= MOD
    print(ans)


if __name__ == "__main__":
    main()

# ARC058D - いろはちゃんとマス目 / Iroha and a Grid (ABC042D)
def get_fact(lim):
    # compute a toble of factorials (1-idx)
    fact = [1] * (lim + 1)
    x = 1
    for i in range(1, lim + 1):
        x = (x * i) % MOD
        fact[i] = x
    return fact


def get_inv(lim):
    # compute a toble of inverse factorials (1-idx)
    inv = [1] * (lim + 1)
    inv[lim] = pow(fact[lim], MOD - 2, MOD)
    x = inv[lim]
    for i in range(lim - 1, 0, -1):
        x = (x * (i + 1)) % MOD
        inv[i] = x
    return inv


def comb(n: int, r: int) -> int:
    # compute nCr (n! / r!(n - r)!)
    return fact[n] * inv[n - r] * inv[r]


def main():
    global MOD, fact, inv
    H, W, A, B = tuple(map(int, input().split()))
    MOD = 10**9 + 7
    fact = get_fact(H + W)
    inv = get_inv(H + W)
    ans = 0
    x, y = H - A - 1, W + A - 2
    for i in range(B, W):
        ans += comb(x + i, i) * comb(y - i, A - 1)
    ans %= MOD
    print(ans)


if __name__ == "__main__":
    main()

+    x, y = H - A - 1, W + A - 2 -        ans += comb(i + H - A - 1, i) * comb(W - i - 1 + A - 1, A - 1) -        ans %= MOD +        ans += comb(x + i, i) * comb(y - i, A - 1) +    ans %= MOD