Last time I checked, many banks start looking at your rentals as more of a portfolio once you have 4 properties. They no longer expect that your personal income will cover all of your debts and they count your rental income differently. Of course, it has been a while since I had 4 properties, and things have tightened up since then. For instance, on May 1st National Bank changed the way that they calculate the portion of rent that they will consider verifiable income, suddenly making it much more difficult to qualify.
In a nutshell, all banks have their own policies and an experienced mortgage broker that specializes in investment property is your best bet. And always make sure that your rent is at least 1.2 times your basic expenses (mortgage, taxes, heat, and possibly insurance).