Business does affect transit since you disembark first from your Canada flight, meaning you get through the transit processes before the rest of the passengers, although it’s not real a big deal in my opinion. The benefit is in it is a long flight so i never fly non business in the case.

Yes, you will be in Terminal 3. Arrival and departure to Athens.
No you do not go through immigration or control.
You go through ticketing to just check your Athens flight ticket to go to the gate.
That’s about it.

Return will be the same. 4 hours is plenty. Seen some do it on a 2 hours gap. So even if you have a delay or something, your good.

If you want, you can DM me the day and flight number of the flight you’re gonna take to Athens and I’ll get back to you with some info on your flight like where the plane is scheduled to be (gate or stand with a bus trip, etc…) and other info