You are considering 6 destinations in 14 nights. That is rushed, especially with 4 people. For each change, you have to pack your bags and check out of the hotel, get to the train station at least a few minutes before departure (much more if you need to wait in line to buy tickets) to find your train and board it, then get to to your next hotel to check in and leave your luggage before beginning any sightseeing. Even a 1-2 hour train ride translates into nearly 1/2 a day of travel time.
Are you departing on April 11? If April 10, then it's 13 nights. When is your departure flight? It is usually best to spend your last night in the departure city. With the last night in Milan, we're up to 7 destinations, or 2 nights per destination, giving you one full day at each. Another question is whether you're going to Italy from somewhere nearby, meaning a short flight and no jetlag, or are you on a long-haul flight with a transfer? Are you going to be sleep-deprived and jetlagged? That is likely to slow you down on your first day or two.
After all that, I recommend you omit at least 2 destinations. Since late March-early April is a poor time to visit the Cinque Terre, and since it takes a long time to get there, it's a good candidate. Another obvious one is Sorrento, since it is so far away from the others and also takes a long time to get there.
Even with 14 nights and no jetlag, Venice > Florence > Rome > Milan is a pretty full itinerary. You can visit Verona or Padua on the trip from Venice to Florence, taking an early train out and checking your luggage at the train station for the day. You can day trip to Siena by bus from Florence.