Yes, there are lower houses as referenced in several side quests (although I didn't keep a list of names or anything, so sorry about that).
Many of them, actually. Not so sure about the serving part, but they definitely try to impress members of the 4 higher families and kiss their noble behinds to gain more influence and stuff like that.
From what I get, each of the higher families is in charge of a certain 'organisation' or area, so for example, Dzemael and Durendaire lead the rebuilt of Falconnest, the house of Dzemael is responsible for law-keeping inside Ishgard, and so on.
And appearently, the lower houses serve as knights under the leadership of the higher houses.