Commentary for Bava Batra 1:4
כֹּתֶל חָצֵר שֶׁנָּפַל, מְחַיְּבִין אוֹתוֹ לִבְנוֹתוֹ עַד אַרְבַּע אַמּוֹת, בְּחֶזְקַת שֶׁנָּתַן, עַד שֶׁיָּבִיא רְאָיָה שֶׁלֹּא נָתָן. מֵאַרְבַּע אַמּוֹת וּלְמַעְלָה, אֵין מְחַיְּבִין אוֹתוֹ. סָמַךְ לוֹ כֹתֶל אַחֵר, אַף עַל פִּי שֶׁלֹּא נָתַן עָלָיו אֶת הַתִּקְרָה, מְגַלְגְּלִין עָלָיו אֶת הַכֹּל, בְּחֶזְקַת שֶׁלֹּא נָתַן, עַד שֶׁיָּבִיא רְאָיָה שֶׁנָּתָן:
If a courtyard wall [of partners] fell, he (i.e., each one of the partners) is required to rebuild it until (a height of) four cubits, [this sufficing to prevent "gazing damages"]. He is deemed to have given unless it is proved that he has not given. [i.e., If one claims half the expenses and the other says that he has already given his share, he is believed, unless the claimant can bring witnesses that he claimed and the other did not give. For all know that each must give his share, and the other would not have built alone (without the first having contributed), but he would have brought him to beth-din.] From four cubits and above, he (the second) is not obligated (for half the additional height). If he adjoined another wall to it, even if he did not roof it, all is imposed upon him. [i.e., If after the first built well above four cubits and the second did not wish to assist him in it, he (the second) adjoined another wall to it in order to roof it from wall to wall — even if he had not yet roofed it, all is imposed upon him, he having made it evident that he favors the additional height.]
Bartenura on Mishnah Bava Batra
English Explanation of Mishnah Bava Batra
[If the fence was built] four cubits or higher, they do not obligate him [to help in building it.] If [the one who did not contribute] built another wall near it, even if he did not put a roof upon it, they obligate him to share in all of the costs. He is presumed not to have paid [his share] unless he brings proof that he has.
Mishnah four continues to discuss a wall built in a courtyard owned by two partners, a topic that began in the first mishnah of the chapter.
In the times of the mishnah, a standard wall, built in a courtyard was about four cubits high (over two meters, a little less than seven feet). Therefore, if the previous wall fell down, each partner would be obligated to rebuild the wall until it was four cubits high. If Reuven claims that Shimon did not pay his share, Reuven must bring proof. Since this law is commonly known, we assume that Reuven would not have begun to build the wall until Shimon paid his share. If he did build the wall without first collecting, he must bring proof that Shimon still owes him money.
In contrast, if Reuven built the wall over four cubits, he cannot ask Shimon to pay for the costs of the added height. Since walls are not commonly built over four cubits high, Shimon can claim that he did not want such a high wall. If however, Shimon were to use the wall to support another wall, and show that he intended to lay a roof on the two walls, he must pay for the added height. Although he was not initially obligated, since he subsequently used the wall, he is obligated to pay for it. Since this law is not commonly known, we cannot assume that Shimon paid his share. Until he brings a receipt showing that he has paid for the wall, he will still be obligated towards Reuven.