I'm going to assume you meant a 4 to 6 unit apartment building rather than 4 to 6 bedroom apartment.
In the real estate industry there is a large divide at 5 units. Single family to four unit is residential and is covered by different lending, pricing and disclosure laws. Five units and up are in the commercial realm and disclosures and pricing and many other aspects change.
We teach investors such as yourself and your husband how to look at the numbers to make the correct choices for you. The most important number you will need to see is the NOI which stands for "net operating income".
San Francisco has many markets that are good investments but for different reasons. I have a building listed now in the Downtown area that is 27 studio units. The gross rent multiplier (GRM) is 16 and we will deliver the building at a 5% capitalization rate (cap rate). This building is a good investment because it is comprised of studios that turn over well and in a rent controlled city that is what you need to keep market rents. Larger units have tenants that tend to stay in the unit long term. So efficient building with good rents.
Other markets in SF are more about pride of ownership. Take the Marina or Telegraph Hill where the GRM is in the higher teens to low twenties and the cap rates are in the threes and fours. These buildings may appreciate a little better and you might be able to lower the GRM and raise the cap rate if you can reduce costs.
It is a much larger subject than can be set out in this forum. Basically there are three plays, reasons to invest, appreciation, cash flow and tax benefits. What is right for one investor may not be right for another investor.
If you want to learn more look into the different meet-ups that are put on by people that can give you the how to's and an understanding of the tools you need to know.
As for areas of the Bay Area - the strongest rents are down in the South Bay where the job centers are. I would also stick to geographically contrained areas near the jobs. Sunnyvale, Santa Clara, Campbell are strong contenders. All next door to very expensive neighborhoods and near jobs.
It is also important to consider whether better cash flow today on a 5 or 6 unit building will outweigh the capital appreciation of a fouprlex.