The photos in this post show how to integrate multi-family housing into single-family neighborhoods. The two-story duplexes fit in easily and look like two-story single family homes. These buildings fit here partly because they are well-designed and consistent with the architectural character of the neighborhood. They also have the same front yard and side yard setbacks as the single family homes as well as detached garages. It's possible that a four plex of a similar size would also fit in easily, but that would depend on the neighborhood. There's no good reason to start inserting too much density into a single-family neighborhood when there are other parts of the city that can more easily accommodate it. The images here just show that, if the community supports it, adding some modest density in single family neighborhoods can be done well.
Cities planning new subdivisions should consider this sort of mixed density as a way of integrating different economic and social groups into neighborhoods. Definitely a better idea than large apartment complexes cordoned off from single-family by arterial streets and better than plopping a large apartment building right next to a single family home.