Agreed. 4.5 hrs is not really a lot of time - at most you'll get about 1.5 hrs of practical time to visit. You could do Parliament Hill / War Memorial / Wellington in that time - but not much else.

Another option - given that fall colours are peaking this week, is to do a driving tour around the city's parkways and attractions - Colonel by / Queen Elizabeth parkways, Wellington / Parliament, Sussex, Rideau Hall, Rockcliff Parkway and maybe even a bit of the Gatineau Park if there's time. Lots of magnificent views especially with all the fall colours. See if you can negotiate with a taxi at the airport for a fixed rate for the 2h of driving.