I need to generate 4 sitemaps which differ in content included in them according to certain taxonomies! XML sitemap can generate one sitemap. When I try to generate other it displays error: A sitemap with the same context already exists.
I suggest installing the Views Data Export module. It will allow you to create XML pages from your taxonomy and as many as you want.
I would create one view and 4 page displays with the varying filters to generate your pages. Then change the output to XML.
I looked over the XML Sitemap, and the error you are experiencing seems to be something they are trying to troubleshoot. You can follow along there, or try some of their methods.