private void render_4d(int opcode, double p1, double p2, double p3,
                         double p4) {
    RequestOutputStream o = display.out;
    synchronized (o) {
      begin_render_request (o, opcode, 36);
      o.write_double (p1);
      o.write_double (p2);
      o.write_double (p3);
      o.write_double (p4);
    }
  }