Come Christmas I am booked to fly SEA-YYZ-JFK on two separate tickets. (and two airlines: UA & AA) Can I assume that I will have to go through immigration, claim baggage and customs (unless pre-cleared?), then change terminals and do the standard check-in routine? I have a 4-hour layover to hopefully lower the odds of a missed connection.
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):
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.)
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.