Good base location for 4-6 nights in Emilia-Romagna?

I presume that you will be relying on public transportation (not a rental car) for your trip.
On this basis, given the locations you want to visit, I strongly recommend you stay in the historical center of Bologna. I understand your desire to stay in a smaller place, but the historical center of Bologna is compact and will give you the feeling of a smaller city, similar to the others you mentioned. The advantage of staying in Bologna is from the transportation logistics standpoint. Bologna is the most important rail hub of Emilia, and it is central to all the main places worth visiting in the region: Ferrara, Ravenna, Forli (Borgia), Faenza, Parma, Reggio, Modena (even Florence in Tuscany in fact).

Trust Roberto on this. We stayed a few days in Parma with a day trip to Bologna. Wish I would've done the opposite. Loved the region, btw.

Bologna. One of the best of the larger Italian cities. And indeed a perfect base for visiting all those smaller cities around.

Another vote for Bologna. I did day-trips to both Faenza and Ravenna from Bologna last summer. Very easy. Bologna's historic district is very nice.

If you can't bear the thought of staying in a large city, I will say that I thought Ravenna was particularly attractive, but for some reason not tourist-overrun. But it's not particularly well-located as a base for side-trips. Rome2Rio indicates that there are just 5 buses a day between Ravenna and Faenza. Trains run more often (about hourly), but a transfer is involved. Ravenna to Ferrara would require changing trains in Bologna.

Haven't been to Ferrara so can't compare it to Ravenna. To get from Ferrara to Ravenna you'd take a train to Bologna and change. There seem to be some direct trains from Ferrara to Faenza, but there are also some 2-hour connections, so check the schedule carefully.

For what it's worth, I stayed three nights in Ravenna in summer 2015 and loved it. Best part of the stay was a day-long mosaics workshop at Koko Mosaico. I did the one-day "short course" and LOVED it - instructor was one of the two owners of the studio and, as I was the only student that day (in August), I had a private day-long amazing course in creating Ravenna-style mosaics.

Bologna would be the best location logistically, and it's a very pleasant city. The other three make good daytrips.

If you were to split your time with two bases, either Ravenna or Ferrara would be fine as well, in addition to Bologna.

I agree that Bologna is a good choice if you want to visit other places. We had plenty of time and spent 3 days in Ferrara (I ran a half marathon) 3 days in Parma and 1 in Bologna. I would've liked another day in Bologna. We liked all 3 but Ferrara was a bit overwhelming with SO many bicycles everywhere. Parma was my favorite. Great for running and also we enjoyed visits to cheese, balsamic vinegar and a parma ham place. Going back to Bologna this fall and to Ravenna (another half marathon).

Last fall my 2 cousins and I spent several weeks in E-R. Originally I booke an apartment for a month in Bologna. My 2 cousins stayed in Bologna for 2 weeks before I got there and I was remaining in Its,y for 2 weeks after they left. They found Bologna a perfect base. When I fot there we move to Parma for a week, and it was convenient. I decided to rent a car in Parma and for 3 people it was very convenient and our hotels had parking. After a week in Parma, we moved for another week to Faenza to spend mor time in the eastern part of ER. After they left, I stayed in Bologna for a few days, but was glad for a car (I had an apartment in the ZTL with parking). I had knee problems at the time and while all cities have train service, Bologns's is more frequent to the cities throughout ER.... But the car was my savior.

Highlights of my trip: a private tour of a paramsan cheese factory organized through the cheese consortia and a tour of a Balsamic Vimegar place. Both free, but require transportation, but could be done with cabs.

Thanks, everyone! It does sound as if my style of train travel (spontaneous) would work a lot better from Bologna.

Even with Bologna as the rail hub of the region, check schedules carefully. I was there almost 20 years ago so things could have changed, but I remember that for daytrips I often had to get up early to get a train. Otherwise, I would have arrived in time for lunch, having missed much of the morning. For instance, there might be a train from Bologna to Parma at 8:30 AM and 11 AM, but not in between.

Great! Now considering a stay in Bologna, with day trips to Ferrara and Faenza, followed by a couple of days in Ravenna, including a mosaics workshop. Thanks to Harold's advice, I'll try to be more careful about train schedules. Thanks again!