Depending on the lot, there could be many configurations to divide the outdoors space.

You may be able to get the city to allow it as a non complying triplex depending on how long it’s functioned that way, what the current configuration is, and whether or not from a building code perspective the units are have the correct fire separation.

I had a riverfront house with an attached garage apartment and could be configured several different ways; I had different listings for each size (to minimize confusion regarding how much of the house a guest was getting), and made it so you could book the whole house furthest in advance, then for closer stays there was the option to book the smaller version.  

We have always gone to 200 amp but then again 300 amp would definitely be a plus for a geothermal configuration, or the 240v 3x50amp breakers for a LARGE on-demand HW system or maybe the HVAC of a discreet 'grow' facility :) Admittedly the extra amps would adequately supply the future ADU BUT assuming it's allowed I would be inclined to install a separate meter/feed for that.The $3500 sounds reasonable...the last 200 amp upgrade I did myself ran about $800 in materials as those 'code required' AFCI beakers will 'break' you $$$.

Look around the neighborhood to determine how other properties are configured (bed/bath & square footage).

Just to make sure the construction and building configuration you have in mind isn't in conflict with setback or other restrictions as Luke mentioned.

In the homes that you have used this strategy, what was the bedroom/bathroom configuration? 

Depending on your investable capital 4-unit MFH in community configuration could be a good allocation as it allows better capitalization for supporting services (PM, Leasing, Maintenance). 

As a current basement dweller and someone who slept in a converted living room for years in the NOVA area, I have somewhat of an idea about room rents in the area, but I've realized that there's a lot of discrepancy in advertised rents on Craigslist without a way for me to have a reliable indicator of how much rent my property/room configuration could get to calculate if house-hacking would be beneficial.

There are so many errors I don't even know how to configure a way to explain why.