Is there a tourist pass that may be cheaper that covers entry to major attractions in Rome, Florence, Venice, and Milan for a family of four (including two teenagers)? If not and individual tickets need to be purchased what are the attractions in each of the city that are a must see?
There is no short or easy answer for your question except to say that there's no "one size fits all" when it comes to must-dos. Example: If I was to answer from my own point of view, my list would be very heavy on art museums, churches (for the architecture/art) and some Roman archeological sites. But I see from one of your prior posts that "we are not much into museums and architecture", and that suggestions for some of the more popular attractions - and the need to acquire advance tickets for most - were covered in a previous thread:
https://community.ricksteves.com/travel-forum/italy/trip-to-italy-june-2025
So your best bet at this point is to get yourselves a couple of good guidebooks and spend some time browsing what appeals for sightseeing opportunities in each of your destinations. One positive point is that your teens will enjoy free entry to many museums if they're under the age of 18: always carry their passports for proof (and you need to carry your own as well). Check the details of each attraction you are interested in to see what their ticketing policies are.
No, there really isn't a tourist pass for each city that's going to save you much for $$. As well, you still have make advance, timed entry reservations to the most-visited attractions that are covered by a pass. This is true of the Colosseum and Galleria Borghese in Rome, and Uffizi and Accademia in Florence. The Vatican Museums (not covered by the Roma Pass, and under-18 teens will enjoy reduced, but not free, entry) also requires advance, timed-entry reservations to avoid standing in a VERY long queue.
Hopefully someone else can answer for Venice as it's been too long since last I visited that one, and we didn't do anything in Milan last time that involved tickets/entry fees.
A pass for 4 different cities for a whole family? No. I would suggest getting a guidebook on Italy and reading up on what you want to see in each city, depending on how many days you will spend in each. As a start, look at the At A Glance page for each city in the Italy section of the Explore Europe subforum: https://www.ricksteves.com/europe/italy
What are your interests? What made you decide on Italy for vacation? History? Picturesque scenery? Food? An interest in wine? Are you outdoorsy?
As mentioned you need to put in the work to see what interests you. Your kids are old enough to give their input... mine did for our first family trip. After that then look at how to get tickets etc.