I have a 4.5 hour layover in Montreal, on my way to Europe. Is that enough time to leave the airport and have a quick glimpse of the city? As an American citizen, is there anything similar to TSA precheck to make the line back in quicker?
Depending on your airline, you may or may not be able to stay airside when connecting. However, if you chose to leave the airport, you would need to clear customs to enter Canada, which would take additional (potentially significant) time.
In terms of the trip to Montreal itself, it would minimally take about 30 minutes each way by car to get from Montréal-Pierre Elliott Trudeau International Airport (YUL) to somewhere interesting like the Old Port of Montreal. If you wanted to give yourself some amount of time to do anything there (say 1 hour), and a bit of wiggle room for traffic (say 30 minutes), and then time to get back through security (let's say another hour), you're looking at 3.5 hours alone. You also need to give yourself enough room to get to the gate before the doors to the plane close, which depends on the airline, but 10-15 is the absolute minimum.
Sum that all up, and you definitely don't have enough time to comfortably make a trip. If you had maybe 6 hours, or didn't have to clear customs, I might consider it. But there are too many risks of significant delays, particularly in traffic, customs and security lineups that add up to making this short of a layover not a good idea.
To answer your question about a TSA PreCheck equivalent, if you have a NEXUS card, you would be able to use the express line through the security checkpoint. This would also grant you expedited customs clearance when entering Canada, and access to the Global Entry lines when re-entering the United States, and access to TSA PreCheck.
A glimpse is possible within that time although you certainly won't be able to see much.
Assuming an on-time arrival and that your luggage is checked through, you can exit the airport fairly quickly. As anyone exiting the airport after arriving from another country, you need to do exit immigration which is generally quick, particularly for Americans. Figure about 15-20 minutes to deplane and arrive that immigration and then it rarely takes more than 15-30 mins unless arriving at particularly busy time.
Once exiting from the secure area, you walk less than 5 mins to the taxi stand and wait another 5-10 for a taxi which takes 25-40 minutes until downtown, depending on traffic and outside of rush hour. Even in rush hour, the trick to take a taxi to Metro Angrignon which takes 20 mins and then you can will be downtown in the old for in 22 minutes from there. Totaling all this gives you up to 1:45 to get anywhere central.
For the return, use the same path, either taxi or metro followed by taxi which brings you to the airport in 45 mins. Nexus is available to get you back into the secure area quickly and Canada does not have exit immigration, so you can get to the gate within 30 mins. Then you have to be there early enough for boarding which happens at 30 mins to departure.
This leaves you exactly an hour downtown with virtually no margin for getting lost, stuck in random traffic behind construction, a snow plow, the metro breaking down or anything else that happens in Montreal.