Connecting through Toronto on separate tickets

Yes, your assumption is correct. Depending on the time of your flight usually the customs usually is fast. However, you have to keep in mind the Christmas rush and be swift at the airport. You also have to keep in mind that you will go through preclearance when boarding a US bound flight. If you/r bags aren't pulled over for extra scrutiny you will be fine, I think.

Pearson has a very cool Connecting Guide that answers this question for you. I entered your details, and it said (bolding mine):

1. Arrive at Toronto Pearson. Your flight will be arriving at Terminal 1.
2. Confirm your gate number. Once you're in the terminal, look for your departing flight on the flight information screens then follow the "A" connection signage.
3. Go to Canada Customs. Look for the A gates signs and follow them. This can be a bit of a long walk, so take advantage of the moving sidewalks.
4. Pick up your baggage. Follow the signs for Baggage Claim and pick up your baggage. You'll need to take it with you to recheck it.
5. Recheck your baggage. Drop off your baggage at the connection drop-off belt.
6. Take the LINK Bus to Terminal 3. Your connecting flight will be leaving from Terminal 3, and the quickest way to get there is the wheelchair accessible Terminal Link. The bus picks up guests 24/7 from the Departures level curb of Terminal 1 and travel time is approximately 6 minutes. Follow the A gates signs to the bus stop

At this point, things get confusing because it has already told you to drop off your baggage and now it wants you to pick it up again (I think connections USA-USA are rare. Take instructions from someone locally about whether your baggage goes on the connection belt or goes with you to Terminal 3 for your conversation with US customs. My guess is it stays with you, see below.)

1. Pick up your baggage. Follow the signs for the A gates to the connections carousel and pick up your baggage. You'll need to take it with you to recheck it.
2. Go to US Customs. Look for the US Customs and Border Protection signs and follow them.
3. Drop off your baggage. Drop off your baggage at the baggage drop-off belt.
4. Go through security screening
5. Head to your departure gate. There are various flight information screens throughout the terminal that will point you in the direction of your departing gate.

You can find interactive maps of both terminals (AA uses Terminal 3, United uses Terminal 1) on the Pearson web site. The only possible confusing part is the double customs. You will need to clear into Canada, and then preclear into the US while still in Canada.

Longest lines I have ever been in have been up to an hour for customs - this is why I got Nexus - but in Terminal 3 they have a new automated system that is much faster for arriving from the US. A more normal customs wait is 10 or 15 minutes. Security can be no wait at all or 30 minutes in busy times. Baggage dropoff might be another wait if you don't just toss it on the connecting belt. Since you're changing airlines I think you need to go talk to a human and get a new tag. That can be a 30 minute wait if you have no status with the airline you're lining up for.

If I was doing this with a 3 hour connection I would be nervous. Probably not with real justification, but I would still be nervous. With a four hour one, you're fine. If this is something you'll do regularly, get Nexus/Canpass/Global Entry. They will save you a ton of time.