I have something that is bothering me. In Eruvin 5A (see Outlines to the Daf, 1:a:1), I don't see why, if a Korah is a Mechitzah, the platform must be four just because Mechitzos are not made for less than four. The Korah is only a Tefach thick, so the platform doesn't make it thicker. I welcome any answers you have about this.
See RASHI 5a DH u'Man who explains the opinion that a Korah makes it permissible to carry in a Movui because it is considered as a Mechitzah. Because the platform is less than 20 amos away from the Korah, the outside point of the Korah can be considered to descend and to make a Mechitzah to surround everything in the Movui. However, if the platform is less than 4 wide, it follows that the Mechitzah is not surrounding an area which is under 20 amos high and which has a width of 4. Therefore, it is not considered a Mechitzah. The Korah is only a Tefach thick and the platform certainly does not make it thicker, but that does not make a difference because the problem here is not the width of the Mechitzah (one does not require a Mechitzah of 4 thick). Rather what one requires is that the Mechitzah should surround an area which is big enough, i.e. 4, and if the platform is less than 4, the Mechitzah is not surrounding a large enough area.