A lot of Assangers sources against Clinton are highly questionable at best. That being said, yes there are four candidates...but Stein and Johnson (aka the two candidates not named Clinton or Trump) are...well they are laughably bad (in different ways.)

However there is another part of the election you have to take into account. America runs what is known as a First Past the Post system. That is, unlike say proportional election, the matter is who finishes first. In such a system, the two other parties that are NOT the Democrats and the Republicans have virtually no chance of winning. More, it opens up a 'split vote', in which candidates who feel strongly about one side of the political spectrum can undermine themselves by not pooling their votes behind one candidate. This is often while people say that if you vote for a Third Party Candidate in the US election, you are throwing your vote away.

I personally don't think Clinton is any more corrupt than any other career politician and the leaked emails, after being sifted through didn't really show me anything shocking. She's a good deal LESS corrupt than I actually thought at the beginning of the election cycle. She's far from perfect to be sure, but if I was an American I would feel comfortable putting my vote behind her.

...Trump on the other hand terrifies me because I think he is unfit in almost all ways from qualified to having the temperament to be the head of the worlds only remaining super power. I don't want somebody with as little self control, self awareness and who honestly asks "Why can't we use our nuclear weapons?" anywhere near the White House.