Yes, CD-RW cannot be written faster then specified. There may be some out there that can but CD-RWs use phase changing surfaces so now instead of brning the surface the writing depends on how fast or how slow the surface can heat up and cool down and what burn will result from that. So as a result the 4X CD-RW would cool down and heat up iln some time that is too slow for faster burning and you may get just a useless CD with random bits.