How did he lose 4 million votes? Most of that can be found in the DNC emails, among other places. There was a huge media campaign smear against him (led by the DNC itself), massive amounts of donations were funneled in ethically questionable manners into the Hillary campaign, stealing money from state tickets. People were hired in a black flag op to discredit his supporters.
I'm sure the list goes further.
However, I do think one of his biggest issues was that he wasn't a member of the team. I saw it first hand at the caucus; members making references to the fact that she was a life-long democrat, as if it was actually important. Republicans certainly got over that quickly.
Still, if the DNC had played fair with Bernie, I think that two million votes flipping isn't so far fetched.