Next time, I'd recommend going to the airline counter at the airport to process the cancellation.
This is terrible customer service though, and i'm quite surprised they're not willing to refund after a 4 hour delay. Before initiating the chargeback, I would check the RJ Contract of Carriage to see whether a 4 hour delay qualifies for a refund or not. If it does, highlight the relevant section and let AmEx do the rest.