The YC FAQ answers the first three: http://ycombinator.com/faq.html
As for the fourth, of course not.
As for the fourth, of course not.
Please forgive me if the question seems redundant. I am reluctant to even ask as YC has a reputation of integrity. However, the mantra that "ideas are worthless" causes concern that perhaps precautions might not be taken to avoid such a scenario (because by implication, it would seem this could be viewed as a non-event). My apologies if I have misunderstood the views of YC on the matter.
As a counter point, if you want to raise external funding, you probably should accept the fact you will likely have to share your idea without protection - as its unlikely any VC will sign an NDA, and you have no idea who they will push your pitch deck to in order to get a 2nd opinion.