That’s brutal, especially on a ski trip where the whole point of the trip depends on that bag showing up.
Short answer: yes, you should be reimbursed for reasonable purchases and rentals even if the bag shows up a few days later. Under the Montreal Convention, airlines are on the hook for “necessary and reasonable” interim expenses while your baggage is delayed, not only if it’s permanently lost.
Ski rentals, a basic ski jacket and pants, base layers… all of that is pretty easy to justify when you’re literally at a ski resort and otherwise unable to participate. The key is to keep everything reasonable and keep all receipts. Don’t go luxury shopping, just functional replacements.
Once the bag is returned, the reimbursement doesn’t magically disappear. What matters is that those expenses were necessary at the time because of the delay.
Make sure you’ve filed a baggage delay report with AF already, submit receipts as soon as you can, and expect some back and forth. They may try to lowball or push back, but EU rules are on your side here.
It’s still a pain, but you shouldn’t be out of pocket just because they took four days to find your skis.