If income production is the main reason for purchasing then the income is most often the primary method of valuation as supported by the other methods.
To tag-on to PE's post...
In most cases, there are two types of buyers:
Onwer-user and Investor.
The owner-user purchases a property for its utility (use) and the investor purchases its property for its income.
As David pointed out, with small residential income properties (2-4 units) there is the potential for owner-user utility. In other words, the typical buyer for a duplex might be purchasing the property to live in and use the 2nd unit's rent to subsidize the mortgage payment. Many times in a case like this, the buyer will pay a premium for certain aspects of the property that an investor would not. That is because the buyer is going to live in the property and certain features (remodeled unit, large patio area, quiet street, etc.) might be of value to an occupant but might not be of significant value to a tenant. A tenant is unlikely to pay a higher rent necessary to offset the cost of a major remodel. An owner-user will pay an additional price for this upgrade since they receive the utility of its use.
Many times, as the number of units increase, the portion of the typical buyer pool for these properties that are owner-user decrease. An owner-user is much more likely to buy a duplex and live in one of the units vs. buying a fourplex and living in one of the units.
If you are strictly an investor, then you should use the income approach to analyze all potential purchase options. But what is important for you to realize is that if you are evaluating a property that also competes for owner-user buyers, then the owner-user may be willing to pay a premium for the property that an investor will not.
There are some markets where duplexes are primarily investor properties (like areas in Sacramento) and there are some markets where many 4-unit properties have an owner-occupant (like San Francisco). All of the advice I've read is good advice. E. Berry suggests that you identify the ownership (owner-occupied or non-owner occupied) of the property-type you are considering to determine how likely the income approach best reflects the motivations of the buyers/sellers. I agree.