import std.stdio;
import std.conv;
import std.string;
import std.algorithm;
import std.array;
import std.range;

void solve(T)(T h, T w, T n, T k){
  auto last = h * w - 1;  // to 0index
  k -= 1;   // to 0index
  writeln( ((last % n) == k) ? "YES" : "NO" );
}

void main(){
  auto buf = readln.strip.split(" ").map!(to!long);
  solve(buf[0], buf[1], buf[2], buf[3]);
}