The Gemara says, that which we don't allow putting the Eiruv in such a small house, is only an Eruv. But a Shituf (which combines all the courtyards that are opened up to an alleyway) may be put in the small house. Since the Halacha is that the Eruv needs to be put in a house and the Shituf in the courtyard, this small house is no worse than the courtyard. Therefore you may put your Shituf in there.
The Gemara says that this we say the small house cannot combine two cities for T'chum. Even though a small watchman's hut is enough to combine them. That's because that hut fulfills it's purpose (of having a watchman sleep there.) But a small house doesn't do it's purpose (of being an permanent dwelling place.)
The Gemara also says, this which we say that brothers splitting an estate cannot split this house, doesn't mean since there is not enough for two, so one needs to buy the other out. This would be true even if the house was more than four, as long as after the split both sides don't have four, you cannot split it.
Rather it must be in the Halacha that we split a courtyard according to the doors that are open to it. R' Huna says you divide the courtyard to as many shares that there are doors and you give each owner a share for each door he has (since the main point of a courtyard are for people to come and go.) R' Chisda says that each door gets the four Amos in front of his door (so he can load and unload there) and the rest you divide evenly.
Only houses that are four Amos get a share for it's door, since it's fit for living, it's expected to remain there. But less than four which is not fit for living, we expect it to be dismantled, so we don't give a share for it's door.