Air Canada used to be quite flexible about this but a few years ago they changed their policy and I found that check-in agents had been instructed to enforce the "pay to change" policy. Depending on the fare class, the cost of changing a ticket varied from about $25 per ticket to a $100 or so and some tickets were completely unchangeable. The only times I've had any luck since then were in very unusual situations (e.g. when the flight I was scheduled to take was oversold or when there were severe weather problems that were disrupting all flights and peoples' trips to the airports - so that the agents were happy to put people on any flight they showed up for). So, as others have said, it doesn't hurt to ask nicely but don't count on being able to change your ticket.

Depending on flight delays and line-ups for Customs and Immigration, your 4 hour layover might not be as long as you expect.