B+ is more of starting pose than a position. The weight should be centered forward, 100% over the tanding leg. There should be no pressure on the toes.
It's not really a fourth position, because the toes can be poised on the floor behind the standing leg at almost any distance. There is a style variation to this pose. Look at videos and you'll see that the distance varies by the danseur (or choreographer).
The foot can be brushed (like going into an assemble) or placed (like moving the foot through sur-le-coupe-de-pied into a develope.
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