Bava Basra, Chapter 2, Mishnah 4. The opening line of the Mishnah can be misleading to the learner, so we will preface by presenting its main point: if a person has a wall, his neighbor is not permitted to build another wall close to it.
The Reason for the Distance - The Stability of the Wall's Foundations:
We are speaking of a wall standing in an open area, outside a garden, where the ground is relatively soft. In order for the wall's foundation to be stable, the movement of people nearby is required to pack down the soil. If people do not pass by there, the earth will remain soft and will not be compacted, and eventually the wall will fall. Because of this, the Sages prohibited the neighbor from building a wall too close to his fellow's wall, for in such a situation, passersby will not want to squeeze between the two walls, no one will pass there, the ground will not be compacted, the foundation of the first wall will loosen, and its owner will lose his wall.
What is "too close"? Within four amos. The neighbor is not permitted to build a wall within four amos of the first one's wall, even though the land is his own property.
When This Rule Does Not Apply:
This rule does not apply in an urban environment. In an established city, fifty years old or more, the ground is entirely trodden and well-compacted, and there is no longer any reason to worry about shaky foundations underground. However, in places where the concern exists, the halachah applies as stated.
The Case in the Mishnah:
The Mishnah deals with a case where two walls already exist: Reuven has a wall and Shimon has a wall, and they are perpendicular to each other and meet at a corner, like the shape of the Hebrew letter Reish. Now Shimon wants to build another wall parallel to Reuven's wall, in order to expand his area. Although this construction will create the shape of a Ches, a sort of dead-end alleyway with no passage from side to side, and even beforehand there was not much foot traffic there due to the walls meeting in the shape of a Reish; nevertheless, if Shimon wishes to place his wall close to Reuven's wall, he must leave a gap of four amos, so that the movement of people will take place there and the wall's foundations will be properly compacted.
And the wording of the Mishnah: "Mi shehayah koslo samuch lechosel chaveiro, lo yismoch lo kosel acher ela im kein hirchik mimenu arba amos" - Reuven is not permitted to erect an additional wall next to Shimon's wall and parallel to it, unless he distances them four amos apart.
The Law Regarding Windows:
The windows in those days did not have glass panes, but were rather openings in the wall of the house. They used to block the opening with a board, but in the warm months they preferred to leave it open to let in fresh air and light. Hence the concern: one who builds a wall opposite his neighbor's window blocks the light from it, and if the neighbor preceded him, he has no right to do so.
An additional concern is the damage of being seen and the invasion of privacy: a wall built below another's window allows those standing on it to look straight into his house. Therefore, the Mishnah rules that the builder must either raise his wall or lower it, in such a way that those standing on it will not be able to look through the window into the house.
As the Mishnah states regarding windows: "Milemalan, umilematan, umiknegdan - arba amos" - from above them, from below them, and from opposite them, four amos. There are three directions, and they all share the same measurement:
"Milematan" - from below. The wall is lower, and its top is four amos or more below the bottom of the window, so that someone standing on it cannot see inside.
"Milemalan" - from above. The wall rises four amos above the top of the window, so that even someone lying on the wall and leaning over to peek cannot see inside.
"Miknegdan" - from opposite. This is a wall standing directly opposite, which blocks the light. Here too, a gap of four amos (about six or seven feet) must be left, so that light can continue to enter the prior neighbor's window, and he is not deprived of the light to which he is entitled.
In summary: In this Mishnah we learned two laws of distancing. The first - keeping a distance of four amos between walls, to allow foot traffic that will compact the earth and maintain the stability of the foundations, and this law does not apply in an established city where the ground is already well-trodden. The second - the laws of a wall in relation to a neighbor's window: keeping a distance of four amos from above and from below to prevent an invasion of privacy, and from opposite - so as not to block the light of the neighbor who was there first.