I'm Brazilian and here in Brazil there's no well defined culture on what to do with family names. Legally, you can come out with a complete new family name at the moment of marriage, with the only rule that both people should have it.

In my case, my parents already had some long names. Dad 1 2 3 and Mom 4 5. Both me, my sister and my mom end up with 4 5 2 3, I believe because pressure from my grandparents.

By the way, having four family names, like I do, is not very common in Brazil. It is more common to have two, both the last names from the parents, e.g. Dad 1 2 and Mom 3 4 would result in Baby 4 2, with one of the parents optionally taking those names too (traditionally the woman).