You've just missed the big Toronto Ski Swap but Canali was kind enough to post others that are still to come in the S. Ont area. Barrie would be a good alternative, and Pickering & Uxbridge would also be worth a look if they are closest to home for you. A note about the CSPS ski swaps: most of the gear, especially for kids, is actually new, just a season or two old so very nicely priced.
Check out the local shops as well, and you'll find that gear for small kids really isn't that expensive. Skiis & Bikes (Collingwood, Mississauga, Don Mills), Sign of the Skier (Toronto), Sporting Life (I prefer the Collingwood store, but good selection of ski clothing in Toronto), Gates & Boards (Barrie) are all shops that have good selection and good service.
I don't recommend rental equipment for kids at that age because first of all, it can be hard to get properly sized equipment. I've noticed that area rental shops are stocking gear more for kids starting at around the 10-year old age group.
Secondly, the time you would spend going through the rental shop with a four-year old is not going to help get primed for a fun time on the slopes. If they have their own gear, you'll be further ahead by having things better prepared ahead of time, and you'll just need the final dress-up at the hill. You can also practice wearing boots at home: good for you to get used to putting them on and taking them off, and good for the child to get used to the equipment.
If Dagmar is a convenient location for you, go for it. Yes, very small hill, but it has a really good ski school director and good staff. They have a very strong focus on beginners and young kids, so it will be a great place to start.
Mt.St.Louis is another great local hill with a top-notch ski school director who runs excellent programs. Best beginner terrain of all the local areas, nicely separated from the main runs so your four-year old will be quite safe. As Ghost has hinted, you'll have a bit more to ski or ride on yourself at St.Louis while your child is in a program.
There's another hill you'll see an exit for if you're following the 400 to Mt.St.Louis. Keep going the extra 15 minutes to St.Louis.
And after a few seasons, you'll be ready to check out the escarpment areas, and side trips out west or to the Laurentians...