It is forbidden to pass within four amot in front of a person reciting the Amidah, since one who passes in front of him is likely to disturb his concentration. There are those who say that the reason for the prohibition is that a person who is reciting the Amidah is standing before the Shechinah and one who passes before him offends the respect for the Shechinah.
The poskim disagree regarding the question of where precisely is it forbidden to pass. According to the Eliyah Rabbah, the prohibition only applies to passing directly in front of the face of one reciting the Amidah, since only then will passing by disturb him. In other words, if the width of a man is approximately half a meter (1.64 feet), one may not pass in that half meter in front of his face for a distance of four amot (approximately 2 meters or 6.56 feet). However, according to the Magen Avraham, it is forbidden to pass anywhere the person praying can see within his four amot, since passing there is likely to slightly disrupt his kavanah.
They disagree further. According to the Eliyah Rabbah, a person is permitted to come and stand within the four amot of a person praying and remain standing there. This is because he only disturbs the person when crosses through the four amot directly in front of his face. However, if he comes from the side and remains standing before him, he does not disrupt him as much. Still according to the Magen Avraham, it is forbidden to enter into the four amot in front of a person praying at all.