What flight pass are you looking at?

There used to be unlimited flight passes that were only valid for travel on certain days of the week, however I believe these have all been discontinued. Looking at the Flex T&C for some of the passes that have been around for a long time (such as the Western Transborder), the wording you've shown exists but there is no 4-day option to purchase. That wording does not exist on newer flight passes such as the Calgary-Edmonton commuter.

I suspect that AC just never updated the old wording. Unless you're actually looking at a 4-day pass, all the Flex passes should be valid every day of the week.