Your 50% OE (assumed). What is that based on?
Throughout the United States, operating expenses run 45% to 50% of the gross rents (not 40% to 60%).
A fourplex is not commercial and does not have the added expenses of an apartment complex.
That's just plain silly. While apartment complexes may have different expenses, they are not more expensive to operate than fourplexes. If they were, why would anyone in their right mind buy one? To make LESS money? The truth is that apartment buildings and apartment complexes have different expenses, but they also have economies of scale. Those two factors offset each other.
Your operating expenses should be:
Taxes, Insurance, Property management, & repairs/maintenance
(Take into consideration the age of this unit, the older the higher the maintenance % should be)
Oh, how I wish that were true. Unfortunately, it is NOT! Not even close. There are a BUNCH of other expenses, such as advertising, entity maintenance, office supplies, vacancies, legal fees, evictions, court costs, utilites paid by owner (at least during vacancies), damage done by tenants (in excess of the security deposit), capital expenses (not technically an operating expense), lawsuits, etc, etc, etc. (I could go on and on).
The expenses should be calculated on the exact figures, not guess work or a proforma.
Well, that's a great idea! Unfortunately, it can't be done and from your post, it is clear that you don't even know what the operating expenses are, let alone have an exact listing of them.
First, the vast majority of residential properties are owned by individuals in the United States. Very few of the owners of small rental properties have accurate records (I have bought dozens of rentals and haven't see ONE yet). That's one of the reasons the vast majority of new landlords fail. They don't understand expenses or cash flow issues.
Second, you can't know what the operating expenses for a four plex will be this year (which is what's important). How many evictions will you have at 123 Main Street this year? Will someone sue you this year? Will a major appliance go out and need to be replaced?
So, to try to use exact figures on a small rental property is just a waste of time and will be less accurate than using statistical data over a period of time.
Finally, including tax deduction and prinicpal paydown is just silly. A landlord can't eat with principal paydown and full-time landlords usually don't owe or pay income tax, so a tax deduction is meaningless.
Good Luck,
Mike