#!/c/Python34/python
# coding: utf-8


def main():
    [h, w, n, k] = map(int, input().split())
    if h * w > n:
        ans = (h * w) % n + 1
        if ans == n:
            ans = 1
    else:
        ans = (h * w)

    if ans == k:
        print("YES")
    else:
        print("NO")

    return

if __name__ == '__main__':
    main()