fun readLargeInt () =
    valOf (TextIO.scanStream (LargeInt.scan StringCvt.DEC) TextIO.stdIn)


val () =
    let
        val h = readLargeInt ()
        val w = readLargeInt ()
        val n = readLargeInt ()
        val k = readLargeInt ()
        
        val ans = if (h * w - 1) mod n + 1 = k then "YES"
                  else "NO"
    in
        print (ans ^ "\n")
    end