I'm updating my site 3 - 4 times a week but sitemap stats are not changing i mean it's not changing since last 15 or so .
what i mean to say is submitted Urls should be more than 4425+ and indexed urls should be more than 2402. but it's not updating since last 15 days
If you think you haven't done wrong, then you might want to check this out. http://moz.com/ugc/8-reasons-why-your-site-might-not-get-indexed These are the some of the reasons why your site is not getting indexed, hope it helps.
Are you properly sharing your website's updates? Sharing new pages or posts on Google Plus or Twitter will help Google discover the updates quicker..