Hi Brody, great start! The building code in Ontario for adding units is complex - I would recommend either getting requirements from the city like Sherrie said, (in writing), or getting a professional in to look it over.
From a high level, you'll need to make sure you're meeting Ontario Building Code requirements for ceiling height (and allowed bulkheads), window/living space ratios, egress, zoning (you seem to be good there), insulation - as well as, of course, Fire Code requirements for multi-units (fire-resistance between units, fire-resistant closures, sprinklers, fire and CO detectors, insulation between units). Obviously there is overlap and cross-reference between the two codes. If there is a shared mechanical room with a fuel-fired appliance (eg. furnace) then there are more requirements.
It's a PITA, but I would recommend renovating your other units to the 4-unit apartment building standards now, to avoid costly retrofits. This could mean things like sprinklers, or a second drywall layer, or a bigger egress window.
I'm curious, do each unit pay utilities? Central heat? AC?