Advanced settings question

What is the highest value for this advanced setting:

<readfactor>4.0</readfactor> <!– this factor determines the max readrate in terms of readbufferfactor * avg bitrate of a video file.
This can help on bad connections to keep the cache filled. It will also greatly speed up buffering. Default value 4.0. –>

What is the advantage of it being 4.0 by default and not 2.0?

A better explanation of this setting would be great.

Thanks