(Mishnah): If Reuven's fields surround his neighbor Shimon on three sides, if Reuven fenced the first side, the second and the third side, Beis Din do not obligate the neighbor in the expenses.

R. Yosi: If Shimon then fenced in the fourth side, we roll onto him the expenses of the other three sides.

If the wall between the courtyards of Reuven and Shimon fell down, if one refuses to rebuild it, Beis Din require him to build up to a height of four amos. If after it was built, Reuven demanded payment for half the wall from Shimon and Shimon claims that he already paid it, Shimon is believed, unless Reuven brings witnesses to prove otherwise.

Beis Din do not require a person to build higher than four amos.

If Reuven chose to build it higher than four amos and Shimon built another wall of equal height near it and he wants to put a roof on them, even if he had not yet done so, Beis Din require him to share the costs of the extra height. We assume he has not yet paid that extra until he brings witnesses to prove otherwise.

(Gemara) - Rav Huna: When the Mishnah did not require the Shimon to pay for the walls, it was when there were only three walls built. We can deduce though that if Reuven built all four walls, Shimon would have to pay. This is true only if Reuven built the fourth wall in the same way as the first three, meaning that if he built the first three of stone, the fourth must also be of stone; otherwise, Shimon would not have to pay. But if the fourth wall was only made from brick (which is weaker and cheaper than stone), could he charge Shimon for stone? Since brick is weaker, if the wall would fall, Shimon would have to rebuild it!