4 Connections, but I want 1 flight to be in premium... how?

Flying to remote Africa for work which requires 4 transfers from Canada. For the transatlantic portion I would like to be in premium economy since I need proper rest to hit the ground running. The rest of the flights can be economy no problem.

Do I need to book each leg separately in order to do this? I don't see an option on AC website to choose the seat class for each flight (like I can with WestJet). Any insights? Thanks