I recently purchased JMS. I already have four sites built, all in subirectories of a domain and with different databases. However, the general configuration (extensions, etc) are the same. I will be generating many more sites, and I was wondering if there was a way to set up a master site, based off of one of the 4 I have done, and then add the 4 sites as child sites?

Yes you can choose one of the existing website to install Jms Multi Sites on it and convert it as a master.
Concerning the move of the other existing website as children website, this is more complex and can only be done manually.
If you want to convert existing standalone websites as slave sites under JMS multisite, you have to remember that the concept is to share the same PHP code.
So you have to compare manually all the websites to ensure that they have exactly the same extension with the same version.
If the master has all the extensions present the you can experiment to create "fresh slave" site that would be replaced by the existing Db.
You will also have to verify that all images and other specific files are present in the new environment.

So as you can see, this is possible but not easy.
We already did it with billable support for some customer but really, this is not easy.
www.jms2win.com/en/joomla-multisite?page...35&category_id=1