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

void main(){
  auto buf = readln.strip.split(" ").map!(to!int);
  auto lastNum = (buf[0] * buf[1]) - 1;
  auto n = buf[2];
  auto k = buf[3] - 1;
  writeln( (lastNum % n) == k ? "YES" : "NO" );
}