The valuation of a quad is based on comps (sales of similar properties in the last 3-6 mos), just like single family homes.
For multi-family, the valuation is generally based on the NOI "net operating income" which is defined as the total actual annual income from the property minus the total actual annual expenses. Then the formula for the Estimated value is:
Estimated Value = NOI/(cap rate)
Thus if the cap rate is 10% and the NOI is $50,000, then the estimated value is $500,000.
The cap rate will depend on the class of the apartment (from "A" - high end living space & amenities, to "C" - working class functional, to "D" - livable but not nice.) and the economy. "A" has the lowest cap rate (5-6%), B (6-8%), C (9-11%), D (12% or greater). A bad economy tends to increase the cap rate.