Hi Filipe, I just recently butted up against this as well. I took my mortgage broker out for a coffee to discuss a strategy to get to 10 rental properties. My wife and I currently have 4 plus our primary place of residency and it was explained to me that a lot of lenders are not extending mortgages to those with with a certain number of mortgages or above a certain dollar amount. The specific number of properties and dollar amount varied slightly by lender.
His suggestion for us, to get to our goal of 10 buy-and-hold rental properties was to use portfolio lending. He had worked with 3 different lenders who would lend in this way. They still required 20% down but I don't recall him saying that their rates/terms where any different then a conventional financing structure.
Another thing that I am researching is private money but the rates will likely be higher than 4% so may not be the solution you where looking for.
I am in Edmonton so I'm not sure what differences will exist for you in the GTA but good luck and I hope this was helpful.
Elliott.