I am not sure I understand what you mean.
You have 5 blogs that you wanted to keep completely separate.
But you have one main blog with 3 supplemental topics you want to categorize, separate from the other 4 blogs?
Am I understanding correctly? Or do you want to copy the posts into the 4 separate blogs you installed?
If you want to copy from one blog into another installation, I can think of RSS feeds, which you can pull by category
If you want to pull a specific category onto a page called "Photography", as part of the main blog, but with separate sections, you can create a page that uses a template pulling only that category. I have written a post explaining this, see if it does what you are looking for http://www.buddhatrance.com/creating-pages-with-a-separate-loop-in-wordpress
To keep the main loop complete with all categories, and have the "photography" page as a categorized copy of your posts, skip the part that excludes the category from the index, and leave index.php of your theme as is.
This setup works from within the same blog.
Hope this helps