So, I think the biggest fans will realize quickly when starting Season 4 that one of the organized families shown at the very beginning are the "Milligan Concern" - quite obviously sharing the name with Season 2's Mike Milligan.
We are later shown that the "Rabbi" Milligan character (played by Ben Whishaw) joined the Fadda family, so I'm wondering if Mike Milligan is either connected to this character, another Milligan (if any survived), or if the name is just a coincidence - which in a quirky show like this, I could see being the case. However, remember that Mike is ALSO IN the Kansas City mafia.
We know Rabbi Milligan has gone back and forth between conflicting families when younger, so it's not a stretch to imagine him defecting to the Cannon family - could Rabbi just be Mike's father, or something like that? Or perhaps a surrogate father? (Not sure if the timing works - Season 4 is in 1950).
It would be interesting if there was that link as something else tying the seasons together, and perhaps we will find out more or even see related things happen later on this season.