Gemara

41b (Mishnah): If Reuven was taken outside his Techum by Nochrim or a Ru'ach Ra'ah (evil spirit), he has only four Amos [there] to walk;

If they returned him to his Techum, it is as if he never left.

45a (Mishnah - R. Yochanan ben Nuri): If one slept on the road [on Erev Shabbos] and did not realize that it was dark, he has 2000 Amos in every direction;

Chachamim say, he has only four Amos;

R. Eliezer says, he is in the middle of his four Amos;

R. Yehudah says, he may go four Amos in any direction he wants;

48a - Question: R. Yehudah does not argue with Chachamim!

Answer (Rava): Chachamim permit four in both directions. R. Yehudah permits four in any direction he chooses.

Support (Beraisa - R. Meir): He is permitted eight by eight [Amos].

(Rava): They argue about where he may walk, but all agree that he may carry only within four Amos.

51a (Beraisa): "Shevu Ish Tachtav" alludes to four Amos.

52b (Mishnah): If one was traveling and he was outside the Techum when it became dark, even if he was only one Amah outside, he may not enter;

R. Shimon says, even if he was 15 Amos outside he may enter, for those who measure do not put the Techum marker at the full 2000 Amos, due to people who err.

Rishonim

Rif and Rosh (13a and 4:11): Chachamim say that he has only four Amos.

Ba'al ha'Ma'or: Even though regarding one who slept on the road, the Halachah follows R. Yochanan ben Nuri, regarding four Amos of Shevisah, we hold like Rabanan, and we measure eight by eight Amos. I say that the same applies to every four Amos that a person acquires. They are always eight by eight, and he is in the middle.

Rambam (Hilchos Shabbos 27:11): One who left the Techum, even one Amah, may not enter. His four Amos begin where he stands. Therefore, once he left the Techum an Amah or more, he stays in his place and may move only in four Amos, from where his legs stand and outside. Similarly, if one was even one Amah outside the Techum when it became dark, he may not enter.

Rebuttal (Ra'avad): We hold that overlap of Techumim is significant, whether he left for a Mitzvah or due to Ones. If he is clever, he enters the Techum, and once he entered, he entered. Therefore, if he went one Amah outside, he may walk four Amos in any direction, and re-enter! The Mishnah must be like R. Yehudah, and he chose his Amos away from the city.

Magid Mishneh: The Rambam explains why the first Tana argues with R. Eliezer. We hold that overlap of Techumim is significant. If part of where he may walk now is within the Techum he left, it is as if he did not leave. If so, one who walked one Amah outside the Techum should be permitted to re-enter, for he has four Amos! The Rambam explains that his four Amos are in the direction he was going, like R. Yehudah. However, R. Yehudah allows him to pick any direction he wants! The Rambam connotes that since he was walking, it is as if he chose his four Amos in that direction. This is difficult. The Rashba says that the Rambam discusses one who chose his four Amos away from the city, but he should have explained this. Also the Ra'avad explains the first Tana this way.

Rosh (4:11): Chachamim address R. Yochanan ben Nuri, who gives 2000 Amos in every direction. Chachamim give four Amos, i.e. in every direction. R. Chananel's text says 'Chachamim are just like the first Tana!', i.e. the first Tana of 41b, who says that if Nochrim took someone outside the Techum, he has only four Amos. I.e. anyone who does not have 2000 Amos has only four Amos. Why did they need to say that he has only four Amos? It would suffice to say that he does not have 2000 Amos, and we would know that he has only four Amos! It seems that the Rif omitted this, for he holds like the first Tana of 41b, since R. Yehudah and R. Eliezer hold like him.

Poskim

Shulchan Aruch (OC 396:1): One has four Amos wherever he is, even if he left the Techum.

Gra (DH Arba): This is like R. Chananel's text. He rules like the first Tana, for R. Yehudah and R. Eliezer argue according to the first Tana. The Shulchan Aruch rules like R. Yehudah in 405:1 (below).

Mishnah Berurah (2): Moshe said to those who left the Techum "Shevu Ish Tachtav." We learn four Amos from here.

Rema: He is in the middle of them.

Beis Yosef (DH u'Mah she'Chasav u'Pasak): R. Yehudah says that he is in the middle of the four Amos. Rashi explains that he gets two Amos in every direction. The Rif says that the Rif holds like the first Tana of 41b, and the Magid Mishneh says that also the Rambam does. The Tur holds that the Halachah does not follow R. Eliezer, who says that he is in the middle of the four Amos, for he wrote (Siman 405) that one who went even one Amah outside the Techum may not re-enter to be like residents of the city. He has only four Amos. R. Eliezer gives two Amos in every direction. If so, even if he went two Amos outside the Techum, he is still in his Techum! Also the Rambam says that if he went even one Amah outside the Techum, he may not enter. The Magid Mishneh answered this. Since the Rif and Rambam agree, we follow them.

Taz (1): The Tur says "the Rif rules that he has only four Amos, and he is in the middle. The Ba'al ha'Ma'or rules that he has eight by eight." If so, the Rif holds like R. Eliezer. We do not find that he rules like R. Eliezer! The Rosh says that the Rif omitted the difference between R. Yehudah and Chachamim for he holds like the first Tana of 41b! The Beis Yosef proved that the Tur does not hold like R. Eliezer, since he holds that one who left the Techum, even one Amah, cannot return to be like a resident. I do not understand. Surely he has two Amos in every direction, but he cannot be like a resident again! The Rosh said that the Ba'al ha'Ma'or holds that he has eight Amos, and he is in the middle. The Rosh, and [the Tur's] Kitzur Piskei ha'Rosh, mentioned only four Amos when discussing the Rif, but not 'he is in the middle'. I say that 'he is in the middle' [in the Tur] belongs after the Ba'al ha'Ma'or's opinion, like it says in the Rosh.

Magen Avraham (1): The Rema is like the Tur explains the Rif, that he is in the middle of the four Amos. This is difficult. In Siman 349, the Tur discusses two people six Amos apart, and there are two Amos in the middle in which both are permitted!

Machatzis ha'Shekel (DH v'Hu): Tosefes Shabbos understood that the Magen Avraham questions the Tur. This is wrong. Perhaps the Tur rules unlike the Rif! Rather, he questions the Rema, who did not argue in Siman 349, i.e. he agrees that one may choose the four Amos where he wants.

Magen Avraham (1): Rather, the four Amos are in whatever direction he wants. The Rosh says so, and also the Rambam, who holds like the Rif, and the Ra'avad.

Machatzis ha'Shekel (DH v'Gam): Here, the Magen Avraham questions the Tur and Rema. What is the source to explain that the Rif holds that he is in the middle! The Rosh explained unlike this, and this is unlike the Rambam, who holds like the Rif.

Gra (DH v'Elu): This opinion [in the Rema] is like R. Yehudah, according to the Rif and Rosh.

Gra (DH v'Hu): See the Taz. The [Rema's] words "and he is in the middle" should be after "in every direction" [below].

Rema: Some say that he may walk four Amos in every direction.

Mishnah Berurah (9): Eliyahu Rabah says that one may rely on this opinion in practice.

Kaf ha'Chayim (9): The Darchei Moshe rules like Ba'al ha'Ma'or, for we are lenient about Techumim mid'Rabanan. Many hold like the Ba'al ha'Ma'or.

Rema: However, he may carry only in four Amos and their diagonal.

Mishnah Berurah (10): Even the lenient opinion permits only walking in eight by eight, but agrees that he may carry only in the four Amos that he chose.

Shulchan Aruch (405:1): One who left the Techum, even one Amah, may not enter to be like a resident. He has only four Amos from where his feet stand and outside.

Gra (DH me'Amidas): The Rambam rules like R. Yehudah. He chose his four Amos to be in the direction he was walking in.

Mishnah Berurah (2): The Rema (396:1) brought an opinion that he has four Amos in every direction. The Acharonim rule like this, so one who went one Amah outside, even b'Mezid, may reenter. If he left b'Ones or forgot, and all the more so if he left for a Mitzvah, even if he went up to four Amos outside, he may reenter.