Google doesn't always index everything contained on a website. It is only interested in relevant content and for some reason, it considers 4 pages on your site to be content it's not interested in.
You can create a Webmaster Tools account for your website. In this account you can see if there are crawl errors occurring on these pages, or if you have something in robots.txt telling Google to ignore them. There is even a diagnostic tool that lets you read the page as Google would view it and if that's successful, manually submit the page to Google for indexing (once again dependent on Google considering it to be content they want to index).