It should be impossible for her to win re-election in four years. But it also should have been impossible for her to win this year. All the Republicans had to do was run somebody moderately competent. But...
4 years is a long time in politics. GWB was struggling in early 2001: his "compassionate conservatism" platform had been revealed for the religious fundamentalist welfare-slaughter program it always was, he was under fire for making backroom deals with energy-industry cronies and other serial polluters, was clearly stuttering in public appearances, and half the country thought he was just a rich brat who had cheated his way into the White House.
Then 9/11 happened and GWB became Fearless Leader. For a few years his power had literally no boundaries. He walked into re-election.
Hillary will need a bit of luck, but nothing that cannot be solved with a well-timed spat with North Korea or equivalent.
EDIT: uh, why the downvotes?
[0]https://www.washingtonpost.com/news/the-fix/wp/2016/08/31/a-... [1]http://www.pollingreport.com/BushFav.htm