Yosef Karo, Translation:Shulchan Aruch — Orach Chaim 396…
“ The Law of Four Amot That Every Man Has on Shabbat דין ארבע אמות שיש לכל אדם בשבת Paragraph 1- "Let every man reside in his place." From here, we learn that every person has four amot [in which he is permitted to move] in every place, even if he left the techum. We measure this by his forearms ["amot"] . If he has unusually short limbs, we give him four average amot, each of which is six tefachim. Rem"a: These four amot are measured generously, and he is situated in the middle of them. ”
