Siman 349
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א כָּל אָדָם יֵשׁ לוֹ אַרְבַּע אַמּוֹת בִּרְשׁוּת הָרַבִּים שֶׁיָּכוֹל לְטַלְטֵל בָּהֶם. וּמוֹדְדִים לוֹ בְּאַמָּה שֶׁלּוֹ, וְאִם אַמָּתוֹ קְטַנָּה נוֹתְנִים לוֹ אַרְבַּע אַמּוֹת בֵּינוֹנִית שֶׁל כָּל אָדָם שֶׁכָּל אַחַת מֵהֶן שִׁשָּׁה טְפָחִים וְע"ל סי' שצ"ו.
1 The Law of Four Amot in the Public Domain, 5 Seifim: Every person has four amot in the public domain in which he may carry. We measure these amot by his own forearm ("amah"). If his forearm is small, we allow him four average amot, which are each six tefachim (and see below chapter 396).
ב אַרְבַּע אַמּוֹת שֶׁאָמְרוּ, הֵן וַאֲלַכְסוֹנָן שֶׁנִּמְצָא שֶׁהֵם חֲמִשָּׁה אַמּוֹת וּשְׁלֹשָׁה חֳמָשִׁים. וְיֵשׁ מִי שֶׁאוֹמֵר שֶׁמֵּאַרְבַּע אַמּוֹת עַד חֲמִשָּׁה וּשְׁלֹשָׁה חֳמָשִׁים פָּטוּר אֲבָל אָסוּר.
2 These four amot are measured by their diagonal, meaning that he is allowed five and three-fifths amot. There is one who says that from four amot to 5 3/5 amot is exempt but forbidden.
ג מֻתָּר לוֹ לְאָדָם לַעֲקֹר חֵפֶץ מֵרְשׁוּת הָרַבִּים וְלִתְּנוֹ לַחֲבֵרוֹ שֶׁאֶצְלוֹ בְּתוֹךְ אַרְבַּע אַמּוֹתָיו וַחֲבֵרוֹ לַחֲבֵרוֹ שֶׁאֶצְלוֹ, אַף עַל פִּי שֶׁהַחֵפֶץ הוֹלֵךְ כַּמָּה מִילִין בִּרְשׁוּת הָרַבִּים, וּבִלְבַד שֶׁלֹּא יוֹצִיאֶנּוּ חוּץ מִתְּחוּם שֶׁלּוֹ; וְיֵשׁ מִי שֶׁאוֹסֵר וְעַיֵּן לְעֵיל סי' ש"א סָעִיף מ"ב.
3 A person is permitted to remove an object from the public domain and hand it to his friend who is within his four amot, and that friend to another friend, even though the object will move a few millin, as long as the individual does not remove the object from its techum, and there are those for forbid this (see above chapter 301:42).
ד הָיוּ שְׁנַיִם, מִקְצָת אַמּוֹתָיו שֶׁל זֶה בְּתוֹךְ אַמּוֹתָיו שֶׁל זֶה, כְּגוֹן שֶׁיֵּשׁ בֵּינֵיהֶם ו' אַמּוֹת, מְבִיאִין וְאוֹכְלִין בָּאֶמְצַע וּבִלְבַד שְׁלֹּא יוֹצִיא כָּל אֶחָד מִתּוֹךְ שֶׁלּוֹ לְתוֹךְ שֶׁל חֲבֵרוֹ. הָיוּ שְׁלֹשָׁה וְהָאֶמְצָעִי מֻבְלָע בֵּינֵיהֶם, כְּגוֹן שֶׁבֵּין שְׁנַיִם הַחִיצוֹנִים שְׁמֹנֶה אַמּוֹת, הָאֶמְצָעִי מֻתָּר עִם כָּל אֶחָד מֵהַחִצּוֹנִים בְּמַה שֶּׁאַרְבָּע אַמּוֹתָיו מֻבְלָעִים בְּתוֹךְ שֶׁלּוֹ, וּשְׁנַיִם הַחִצּוֹנִים אֲסוּרִים זֶה עִם זֶה.
4 If there are two individuals such that part of one's amot fall in the other's amot, like if there are six amot in between them, they may bring food to the center and eat as long as neither removes something from his area to his friend's area. If there were three people, with a person in the middle, like if the two outer ones have eight amot in between them, the middle person is permitted to [eat with] the outer ones where their four amot overlap with his, but the two outer ones are forbidden from [eating with] each other.
ה אָסוּר לְהוֹלִיךְ חֵפֶץ פָּחוֹת פָּחוֹת מֵאַרְבַּע אַמּוֹת וַאֲפִלּוּ בֵּין הַשְּׁמָשׁוֹת וַאֲפִלּוּ בְּכַרְמְלִית.
5 It's forbidden to carry something less than four amot, even at twilight and even in the karmelit.