Avraham Sacks asks:
Why does a chatzer containing a house less than 4 x 4 amos square not need to contribute to the shituf even though the shituf can be kept in that chatzer?
Avraham Sacks, Ramat Beit Shemesh
The Kollel replies:
The Gemara itself answers your question: On the one hand, it is only people who live in a house who are obligated to contribute to the Shituf, and less than four by four Amos is not considered a house. On the other hand, one can place the Shituf there, since placing a Shituf can be done in a Chatzer as well, and a house that is less than four by four Amos is no worse than a Chatzer.
B'Virchas Kol Tuv,
Eliezer Chrysler