Valuing a 4-plex based on income, most investor would use a GRM(Gross Rent Multiplier) Value/Gross Rental Income= GRM.

I like gross rent multiplier, how much the property produces compared to its costs.  

I usually start with what the average $ per square foot in your area is and multiply that by the sq/ft of my subject, then look at sold properties near the one I'm analyzing and check out what is for sale around there. 

Not looking to get into a long winded debate on this but this is the one I go by:If you multiply your annual expenses by a number between 25-30, whatever number you arrive at is your needed number (higher for those more conservative, lower for those who are less). 

For a quick reference when I run my estimates, I take the bottom floor square footage divide by 49 (as piers are supposed to be no further than 7 feet apart on a single story) then multiply by $350.  

Then, that percentage is multiplied by the amount of principal that is received each year.

After examining income capitalization methods and formulas, you will learn how rates and multipliers are derived.

Dog walkers seem to multiplying every month, and if you're into parks and trails, I have the best one in the  city a football throw away from my door.

In absence of an appraisal, you need to determine the market GIM (gross income multiplier) from comparables and apply this to your projected income.

It is an education on how 10 percent increase/decrease multiply in a hurry when dealing with several units under one roof.