The interest rate is usually much higher for a five unit building than it is for a four unit building. It is ratcheted up to a "commercial' loan instead of a residential loan. This triggers a higher down payment requirement as well. The interest rate and down payment differentials are less pronounced in SF than they are in my Sacramento metro area because most of SF is above the conventional loan limit threshold anyway.
When financing is less attractive for buyers, there is relatively less added value for that fifth unit. - in most areas.. San Francisco seems to be an exception to that generalization.
You could ask your agent in San Francisco to do a more specific analysis than my analysis on SFAR of 100 sold 4 plexes in the past year gave a median of $1,350,000 and an average of $1,411,681 or $337,500 to $352,920 per door. 22 five plexes reported sold on SFAR in the past year averaged $1,677,773 with a median of $1,600,000 or $320,000 per door to $335,555 per door.
A quick survey of the Sacramento MLS shows an average sale price in the past 18 months of 84 four- plexes at $523,000 ($140,750 per door)
The average for five plexes was 601,500 ($120,100 per door)
Another way of looking at it is that the fifth units added an average of only $78,500 to the value of their respective properties in my metro area.
advice? financing is a consideration for prospective investor-buyers; a seller could maybe sometimes benefit by offering some seller financing?