Need help with 4 days of itinerary from Barcelona to Rome

What sorts of things are you interested in?
How long will you be staying in Rome?

Unless you took a flight (maybe on Vueling Air?), going by train would seem the most straightforward. I’d stop in Nice, France. If you wanted to add a second location, maybe Marseille, France or a place in Italy. Milan? Pisa? Florence?

It's a long, long bus ride until you get to Torino (Turin) where you get on trains.

Do yourself a favor and just fly over to Milan or Pisa where you can catch a train. Florence is 1 hour west of Pisa. Another train goes from Milan thru Bologna before getting to Florence.

Trains could work, but sections of the route would take time. Going the other direction a few years ago, it took a very long day, and changing trains along the way, just to get from Florence to Nice. Nice is very nice, though, if you spend any ti d there.

Turin is a nice city that is too often overlooked, but for this trip it would be a bit of detour to go through Turin.

If it was my trip I would do it by train and make a couple of stops along the way. The first stop would be Marseille. The direct train from Barcelona to Marseille unfortunately only runs once daily, it leaves Barcelona 16:34 and arrives in Marseille 21:23. Continue to Nice the next day, spend a night there. And the last night in Genua before taking the train to Rome. If you have a fourth night available I'd probably add that in Nice, but there are many other options as well.

I don't think I'd mess with ground transportation—it's not enough time to enjoy much of what's en route.

I'd consider one of two extremes:
-Fly to Naples or Florence for a few days before a quick train to Rome.
-Add a night in each city and take the 24-hour ferry between them.

Grateful for all the tips!
We will be flying to Nice where we will stay for a few days then take the train to Rome. Planning on having lunch in Pisa 😊