Installed High Sierra and now none of my Numbers documents will open

Have you tried launching Numbers 4.3.1 and opening files via File > New in your menu? Numbers 4.3.1 can open older documents but some older versions of Numbers can't open new documents. Double-clicking a document to open it, instead of going to File > Open, can sometimes trigger an old version of the app that may be in a subfolder in your Applications folder.

Also, simply restarting the Mac often clears up mysterious problems. So if you haven't done that already you should try it. And if that doesn't work try uninstalling Numbers (go to Launchpad, hold down the option key until the icons wiggle and click the x by the Numbers icon) and reinstalling it from the App Store.

SG

Under what Numbers version were those documents created? Perhaps they are too old to be readable by the current version.