The 6px definitely adds space that shouldn't be there on your normal paragraphs within the "chapter" paragraphs... but it shouldn't cause multiple blank lines.
You can try replacing the 6px with 0 (zero) and see if it fixes the issue. I would also take a look at your <br/> css and see if that is giving excess space or a line-height too large. Also check to see if there is some other <div> css that could be adding spaces. One way to see what is effecting a particular section is to use the inspector in Sigil. Just highlight an area and the inspector will tell you all the different css.
Now - for a little advice on the css that is not relevant to your question
div.main does not need a margin-left or margin-right at all... your width is set to 100% so there won't be a margin to divide.
Your p and p.f4 are almost identical; the margin top/bottom in the p should be left for the user to select in their reader/app, and defining the font-name for all body text is forbidden by amazon...so I would recommend something along these lines:
Code:
div.main {width: 100%}
p.f4 {font-size: 1.2em}