open System

let No166 () =
    let IN = Console.ReadLine().Split(' ')
    let mass = (Int32.Parse IN.[0]) * (Int32.Parse IN.[1])
    let N = int32 <| IN.[2]
    let K = int32 <| IN.[3]

    if mass % N = K - 1 then
        printfn "NO"
    else
        printfn "YES"

    ()
No166 ()