A 40 liter backpack doesn't fit the Wizz Air dimensions for free carry-on; no doubt about that. I still believe that there is a good chance that nobody at the gate will care, but I agree with the previous poster that it very much depends on the airport and gate staff in question. Some are strict and will try to enforce; most are relaxed and more concerned about completing boarding in time and on-time departure.

There is no reason to go to the check-in counters as most likely the OP app will be able to get a BP during OLCI. Check-in counters should be avoided if possible.

If it was me with a 40l backpack I would def. take the chance. Most likely it will work. If "caught"/singled out at the gate, just pay and move on.