Shmuel says here 2000 plus 4 garmidi. We held earlier of the din of muvla. Does this mean that across the entire eruvin when we speak of 2000 amos, we mean 2000 + (2, or 4, depending how you learn the din of 4 amos). Seemingly even if you leave the tchum b'da'as you get the 4 amos. so it would always be muvlah. (and then one you went back it would be talui on whether you reaquire teh cities tchumin ...which was the havah aminah of malu gavri and the machlokes was in Rabban Gamliel. And if you do reaquire, then it is always 2000 +.
moshe rubin, bklyn, ny
Shalom
According to the ruling of the Mishnah Berurah (396:8) a person has four Amos in each direction from the spot he is situated, a total of eight by eight. Around those eight by eight Amos he has 2000 Amos in each direction. So in essence one has 2000 Amos +4. Once a person walks out of his Techum even one Amah he is outside of his Techum.
If a person intentionally walked out of his Techum less than four Amos, although his four Amos are still overlapping with his Techum, he may walk only four Amos in all directions. He may not leave those eight Amos (MB 405:2).
If, however, one was forced to walk beyond his Techum or did so accidentally, if he walked outside less than four Amos he may return to the original Techum and regain his original status.
According to the Shulchan Aruch, one has four Amos only in one direction, and his Techum stretches 2000 Amos around those four. If he left his Techum intentionally (even one Amah) he has four Amos only in the direction away from his original Techum. Thus, he may not walk towards his original Techum.
Yehuda Landy