The fourth house represents more than the home, especially in the composite chart. It indicates the basic roots of a relationship - both literally, in geographical terms, and figuratively, in terms of mental and emotional background. The fourth house signifies the innermost depths of a relationship, which may be so far within as to be invisible on the surface.
On top of this, the fourth house does mean collective 'home'. The home is where a couple's deepest emotional and psychological attitudes have their strongest impact. The home is in many ways the physical embodiment of one's innermost being, and is therefore influenced by anything that affects the inner life.
A composite chart with a strong fourth house usually indicates that the two people share their innermost lives and that they probably share their actual place of residence.