The standard Windows driver is active. It restricts paper size selection.
Diagnosis: Confirm the driver. Go to Settings > Printers & scanners > your printer > Printer properties > Advanced tab. Check the 'Driver' field.
Fix: If it says 'Microsoft IPP Class Driver', install the Brother Print Support App. Click 'Get the app' in that window. After installation:
- Select print command in your application.
- Select your printer, click properties.
- In the Print Settings window, go to Basic > Paper Size.
- Select A4 from the list or create a custom size via 'Edit User Defined Size'.
If that didn't work, you may have the app but are changing settings in the wrong place. App settings only apply to documents printed using the app itself.