I'm not sure if the Minneapolis PD has the same policy, but I recall a case in St. Louis where an officer was involved in shooting another officer who was off-duty. The department policy is that officers must answer questions during an investigation. They are not allowed to take the fifth. In that case, the officer refused to answer questions during the investigation, and was fired. He later pleaded guilty to manslaughter and got about 4 years. My guess is that the four officers are refusing to cooperate with the investigation, and thus were fired. That's just a guess, though.
I've also seen cases were officers were reinstated with back pay because they were wrongfully terminated. So, it's also possible that they were fired because the mayor and police chief determined that their firing might calm the protesters. And if they are not all charged, you could expect some of them to try to get their job back.
Still lots of unknowns in this case.