We learned above (daf 2a) that two people who shared a courtyard and decided to divide it can force each other to build a wall to separate the two sides of the courtyard. The Gemara explained that the need for the wall stems from hezek re’iyah – damage done by looking – i.e. a concern with privacy. Not only are we concerned with the privacy when someone looks into his neighbor’s house, but even looking at someone’s field just before harvest may fall under the category of hezek re’iyah. This last situation is explained by the Rambam as being a middat hasidut an issue of piety rather than a real halakha, and he explains that our concern is not so much with damage done to the field, rather the spiritual or emotional harm that is done to the person himself who looks at his neighbor’s field and is jealous of him.
Our Mishna returns to the law requiring that a wall be built, and rules that four amot is an appropriate height to prevent hezek re’iyah. This is true since four amot is higher than the height of an average person, thus solving the problem of hezek re’iyah. Therefore, according to the Mishna, if the wall dividing the courtyard falls down it must be rebuilt to a height of four amot. Both parties must participate in the cost of rebuilding up to that height, but if one party wanted to build it higher, he cannot force the other person to contribute to the work that was done above four amot, since there is no obligation to build it that high.
The Mishna continues and teaches that if the person who refused to contribute to paying for a higher wall then goes and builds another wall that makes use of the existing separation wall, then he has shown his interest in having a wall of that height and he will be made to pay his portion of the wall even beyond the four amot minimum.