One thing to note is that there are a small number of dishes that are "exclusive" to that restaurant. However, all of the 4 share the same galley, so you can actually order exclusives from another. The bulk of the menu is the same across all four of the restaurants (and changes each night), so unless you like the decor of one so much more than the others, it's not really a big deal to be in the same one for the duration of the cruise. You won't be having Italian or French all the time unless you ordered as such.
As well, the "exclusives" are the same throughout the entire cruise.