1. One who carries four Amos in a public domain with a roof is not liable for carrying on Shabbos.
2. There is a dispute about the shape of the beams used in the Mishkan.
3. The opinion that the beams were narrow on top is supported by the verse, "Tamim."
4. The other opinion understands that "Tamim" means that they must be complete beams and not pieces of beams attached together.
5. The "Bri'ach ha'Tichon" ("middle beam") went around three sides of the Mishkan in a miraculous fashion.
A BIT MORE
1. This is because it is not similar to the public domain in the desert which did not have a roof.
2. One opinion is that they were one Amah wide on the bottom, and they narrowed until they were one Etzba (fingerbreadth) wide on the top. The other opinion is that they were one Amah wide on both top and bottom.
3. The verse describes the shape of the beams by saying, "They should be Tamim at their heads," and a verse in Yehoshua says, "Tamu they should be cut." Just as in Yehoshua "Tamu" refers to being finished off, in the context of the beams it implies that they should become very narrow.
4. This opinion understands that the word "Yachdav" ("together") teaches that both sides of each beam should be the same measure, namely an Amah.
5. It is generally impossible for a person to fashion one piece of wood that wraps around three sides of a square or rectangle. However, the Bri'ach ha'Tichon did so miraculously.