While there are similar questions for coming from several wp installations to a multisite installation I have a slightly different issue. I am unsure how to migrate a single site where there are 4 subdirectories (website.com/one_dir, website.com/two_dir, etc) where each directory will become it's own site on the multisite installation. I'm not sure where to begin as the export file from the admin, as well as the db, contain 5 years worth of content not yet divided. Any help is appreciated.
If you've got your posts separated by categories, then you should be able to make an export file for each category.
On the Tools » Export screen, you can opt to export only the posts in a given category.
Make sure you rename each file as needed when you're doing your exports.
Then, once you've got your new Multisite installation set up, and have created your sites, you can use the Tools » Import screen to import your content files to the appropriate sites.