1. Yup, underscores are better 2. No, dashes are better 3. No difference 4. Some search engines prefer underscores, others prefer dashes What You Think?
Dashes are better for SEO. Refer following comment from Matt. http://www.mattcutts.com/blog/dashes-vs-underscores/
I believe the difference is minimal but I have read somewhere on SEO moz that their testing supports that dashes give a slight advantage - personally I think dashes look nicer