
Philips Sonicare ProtectiveClean 4300

Philips Sonicare ProtectiveClean 4300
What do the numbers behind the model names (Philips Sonicare ProtectiveClean 5100, 4500, 4300 and 6100) mean? I can't even find a specific reference to this on philips.ch. It's not about the sound frequency - at least there is no reference to this.