We are planning a trip to Italy in May and want to visit the Bologna area for a week. We want to visit Parma, Modena, Bologna and Ferrara. Any suggestion as to which city would be the best to use as a base? We don't want to switch accommodations each night and would like to use the train to get to the various sights.
I agree, Bologna would be a good base with easy connections to the places you listed. I would add Ravenna, different from the other cities; it was once a capital of the Roman Empire and has several sights with Early Christian/Byzantine mosaics.
Bologna. Find a place to stay that's about 1/2 way between the train station and the centro storico, you'll be able to walk to/from the station for your day trips and be able to rest up a bit when you get back before heading out for dinner (unless you stay for dinner "out"). And please don't skip Ravenna.
Can Ravenna be done as a day trip from Bologna or should it be an overnight?
Ravenna should be an overnight but it can be a daytrip. A short bus ride will take you to Sant'Apollinare in Classe; this church, plus San'Apollinare zNuovo and especially San Vitale complex (Mausoluem of Galla Placidia and the Baptistry) are the main sights, but the time to stroll along to other good sights in between would make a pleasant one-nighter. The city center is very appealing.
Skippable in Ravenna: Mausoleum of Theodoric.
Considering this is your first time through the area, you're better off taking the train. Each of those respective city's attractions are easily accessibly via the train, cars you're best saving for a trip touring around Tuscany and Umbria.
Bologna is a central location and a good base to hub your activities. Plenty of rental and hotel options, along with lively nightlife and plenty of food options. As others have pointed out, find a hotel/location between the main train station and the central piazza.
Ravenna definitely should be on your list to visit. An overnight is a good idea, if you;re unable, get on an early train out of Bologna.
Ferrara is Italy's 'other walled city', also a worthwhile place to visit. Again, an overnight is a good idea, otherwise, catch the early train.
Parma or, Modena....I found Parma to be more appealing between the two. Modena does have Osteria Franciscana, good luck getting a reservation, otherwise, unless you plan on buying a supercar, I found Modena a bit ho-hum.
The first time I went to Ravenna was 2 nights in February, about 1.5 days, and I wished I'd had longer. But if you don't have the time, you can still see most of the mosaic sites in a day trip.
I did those this past September. Def pick Bologna over Parma/Modena to stay for a few nights. It's a bigger city then Parma and Modena but the transportation options were better. I enjoyed wandering around Bologna back streets more then Parma and Modena.
I did the Food Days tour with Alesandro for a day and loved it. I learned about this one from several people on this forum. http://italiandays.it/
We are foodies and we ate at both of these places in Modena. if you can score a reservation at either of these 2 places they are well worth it.
Osteria Francescana - #1 restaurant in the world for a reason. The flavors were amazing and the presentations were so good that that I didn't want to touch them. Massimo came out and talked to all of the tables. - http://www.osteriafrancescana.it/index.php and https://www.bloomberg.com/news/articles/2016-06-14/osteria-francescana-is-named-the-world-s-best-restaurant
Hosteria Guisti - Place has 4 tables and is only open for a couple hours during the day at lunches. http://www.hosteriagiusti.it/