Thank you for your FAST response. I figured out what the problem was -- the developer changed the behavior of the page display in the template. It would allow me to reference the page I wanted to pop up under the J3 template, but under the J4 template, it generated a 404 error.
I ended up calling up the necessary page using a different URL -- and to fix the modal tag in my article, I used your VERY USEFUL DB Replacer tool to change the URL. Great tool!
The thing I haven't figured out -- and it's now WAY too late here to do more work on it -- is why it now pulls the document up as fullscreen width, whereas on the J3 version of the site it was automatically the width of the text. Is that the template or the modal?