The smaller physical resolution that you use in the Display Settings, the larger the objects on screen will become, and that means that an A4 document in Pages may become too big.
First, try the Zoom tool on the Pages toolbar. It may show 100% but has settings for 75 and 25%. Try reducing the document to see if that will pull it into entire page view on your screen.
I have a 32-in 4K UltraFine LG display with this System Preferences/Settings > Display setting:

Pages opens at 100% with a full document visible and plenty of surrounding screen to the Pages application.
If you are viewing this on a TV, visit the When connected to TV section at the bottom of that Displays panel.
Here is the current online version of: Pages User Guide for Mac - Apple Support
If Pages is too difficult for you to use, there are plenty of other choices available including the free Bean word processing product. Like everything else, it does not open Pages documents.