Q: May one sit within four Amot of a person praying the Amida if an object is situated between them?
A: If an object at least 30 inches tall and 12 inches wide is situated in between a person and somebody praying the Amida, he may sit, even if he is within four Amot of the person praying the Amida (Mishna Berura 102:2); preferably, however, one should not sit in front of the person in such a case (Halacha Berura).
SOURCE:
http://www.mishnaberura.com/Default.asp?ChelekID=1&SeifID=347