Assuming your credit score and employment are good, the key issue you'll run into is your debt ratios.The banks and B lenders will use your Gross Debt Service (GDS) and Total Debt Service (TDS) ratios.I usually recommend that people use out the CMHC provided Debt Ratio CalculatorThere is also the government of Canada's Mortgage Qualifier Tool which is quite good.Make sure you enter your T4 reported income into these tools for accurate results.

Most I've worked with will use a % of gross rental income from the lease if the property doesn't have two years of rental history.

For simplicity, if your gross rents for your 4 unit is $1000 ($250 a pop).

Generally 5% of gross rent.

I am a single household person and have a median W2 income of about 42k gross. 

Lets say maintenance is 10% of your gross... so $800 a month.  

At one point I thought the gross income in my business would be good enough to qualify but later found out that they only look at gross income for W2 workers. 

Gross rent is $3,150/mo (average: $30k purchase, $6k rehab, $50k ARV, $785 rent).

Gross rent month 5 $1500.00. 

Many of them charge 25% or more of gross revenue.