> The exception is the B which is tuned to the fourth.

I was going to say it's a major third, but then realized you meant fourth fret.

> And this really only extends the higher range.

No, the palm frets are only under the bass strings. So what it does is allow you to play first position bass string notes at the same time as higher position treble string notes. It extends the possible chord voicings, not the whole instrument range.