There isn't a lot to do in YYC for four hours. 851 gets in at a little after 4pm, so rush hour will be rough! Sometimes it takes me one hour to do a 15 km stretch of the Deerfoot, or even Barlow Trail, both roads which lead to the airport.
by the time you counter in a 20-30 minute possible delay, deplaneing, customs, getting your bags for the connections, etc you'll end up with about a 3 hour wait, keeping in mind you board 20-30 minutes prior to your flight departure, so in fact you might end up with only around 2 1/2 hours to play around with. I would go to the Chilis in concourse A, enjoy some nachos, or maybe one of their burgers or salads, check out some of the retails, which aren't many, then enjoy some drinks in the lounge. By then you'll be ready to take a long stroll to the gate to get ready for boarding.