The Place of a Person — Eruvin 48a
תלמוד בבלי מסכת עירובין דף מח/א
והני ארבע אמות היכא כתיבא כדתניא שבו איש תחתיו כתחתיו [וכמה תחתיו] גופו שלש אמות ואמה כדי לפשוט ידיו ורגליו דברי רבי מאיר רבי יהודה אומר גופו שלש אמות ואמה כדי שיטול חפץ מתחת מרגלותיו ומניח תחת מראשותיו
The Gemara derives from a pasuk that a person's place is four amos.
R' Tzadok HaKohen (Dover Tzedek, Kuntres Ner HaMitzvos, Mitzvas Aseh §1) notes that elsewhere (Berachos 8a) it is stated that from the time of the destruction of the Bais HaMikdash, Hashem has no place in the world other than the four amos of Halachah.
On the basis of our Gemara, Reb Tzadok explains that the place of Hashem is in the soul of a person who observes Halachah. Such souls are spiritual replicas of the Beis HaMikdash. This is indicated by the measure of four amos — the extent of a place of a person.
Reb Tzadok stresses that what is meant here is not the study of Halachah, but rather its observance. He continues on to note that elsewhere (Shabbos 5a) we learn that a person's hand is considered tantamount to a four amos by four amos place. Thus, we see that it is in a person's deeds that his place is determined.
Elsewhere (Pri Tzaddik to Tetzaveh §6), Reb Tzadok writes that Shemiras Shabbos — is Hashem's four amos of Halachah in this world — indicating that the four amos of Halachah are the four amos of Shabbos.
The Gemara in Berachos (25a) states that there must be four amos between a person who is davening and any excrement. Interestingly, the Rashba there writes that the reason why this is the measure of distance is because distancing from excrement is a part of the mitzvah of V'hayah machanecha kadosh (Devarim ). Posits the Rashba: the personal machaneh of each person is four amos, and therefore he must distance himself accordingly so his personal machaneh will remain kadosh.