My suggestion for what it's worth is to see the "Special Needs" desk in YYZ, either have them come to meet you at the incoming gate, or meet them at their desk in Ticketing Hall before you go into the International gates area (not sure if you clear Customs coming in on that route. I am either Transborder or Euro but never both). They can assist with issues that arise out of disability considerations.
They met me and walked me gate to gate in FRA, and I am 100% mobile, just deaf and unable to ask for directions (especially in German), so it seems they will come to you at your PHX-YYZ arrival.
I personally don't think having your luggage make a 4 hour connection is a "special" need -- I think all the bags should do it. Recent experience of my own indicates that is not always the case. Since your concern is accentuated by your physical condition, I'd say don't be a hero and have them do your baggage expediting for you.