I am confused. Doesn't UA 963 go to Newark?

I would fly from BER to FRA to return to ORD. I find FRA easy transit (when you are leaving Europe back to the US) and there are HOURLY flights from BER to FRA. I would get a ticket that went BER-FRA-ORD-XXX.

Unless you wanted to see Frankfurt (and I like it there) I see no need to get to FRA days early. I don't know if you are paying cash or points, but you can buy a ticket on UA that would be Feeder City- ORD-MUC and BER-ORD-Feeder City, the first segment from Berlin to FRA or MUC would just be on Lufthansa. Or it might route you through Zurich on Swiss, or Vienna on Austria. There is no reason that it all has to be on United metal.

There are advantages to having all flights on one ticket.

As for a 4 hour layover, the last time I arrived in Chicago from Zurich, it took 2 hours to get through border control, get my luggage, go through customs, recheck my luggage, and then get to the United Terminal. And that was with Global Entry. So, I would be completely comfortable with a 4 hour layover knowing that its a nice time margin.