My Shift-Command-4 function stopped doing screen shots?

You can't. With the new system those online sites need to provide an extension for the system so there's an entry for the site in the Share menu. Until them use the add or select photo option at the site and when the browser's choose file window comes up look in the sidebar for the Media section at the bottom. Click on it and then one Photos in the right side. Then you can select any photo in the library to upload to the website.

Go to: Apple > System Preferences > Keyboard > Shortcuts tab > select Screenshots > make sure that the box is checked and the correct keys are selected.