import std.stdio,
       std.conv,
       std.string,
       std.range,
       std.math,
       std.algorithm;

void main()
{
    auto input = readln.split.to!(long[]);
    auto H = input[0], W = input[1], N = input[2], K = input[3];

    ((H * W - 1) % N == K - 1 ? "YES" : "NO").writeln;
}