It would be easier to open your existing gallery in JuiceboxBuilder-Pro (assuming you still have a copy of it on your computer), add 4 images via the 'Images' tab and re-save your gallery on the 'Publish' tab than to place new images in the 'images' folder, create your own thumbnails and place them in the 'thumbs' folder and then manually add new corresponding <image> entries to the gallery's 'config.xml' file.
However, you can manually add images to your gallery (as above) if you wish.
After adding new images to your gallery by either method (whether using JuiceboxBuilder-Pro or doing it manually), you would still need to upload the new images and XML file to your web server. It would be easier to let JuiceboxBuilder-Pro generate the thumbnails and the gallery's XML file for you.
I looked all over trying to find where the gallery counter is located, but nada. I think it's in the .js file. But until I change that counter nothing shows. Is there any easy way to update the counter?
Juicebox counts the number of <image> entries in your gallery's XML file. There is no counter to set.
If you do not see the new images in your gallery when you view it in your browser, please see this FAQ:
When I update my gallery I don't see my changes online. Why?