Hi WogOne,
Edit: I forgot about the page margins. You may have to subtract margins from the page size 😊.
Open a blank Numbers document from Template Chooser.
Menu > File > Print (command P) to see the Print Setup Panel.
Printer & Page Size > Choose A4 if it is not already A4.

Page Orientation > Choose Portrait or Landscape.
I chose Portrait (210 mm wide, 297 mm tall).
Click on the "Done" button bottom left of the Print Setup Panel to return to the editable document.
Menu > View > Show Rulers.
From the vertical ruler, drag an Alignment Guide (yellow line) to exactly 0 cm on the horizontal ruler.
(Yes it is strange that the page size is in mm and the rulers are in cm. Ho hum...)
Drag another Alignment Guide to exactly 21 cm.
From the horizontal ruler, drag an Alignment Guide to exactly 0 cm on the vertical ruler.
Drag another Alignment Guide to exactly 29.7 cm.
This will test your patience to get the Alignment Guides exactly at any position 👿.
If you find this too fiddly, you can err on the safe side and set the Alignment Guides slightly inside the page margins.

Insert your background design and resize if necessary to make it fit between the Alignment Guides.
Arrange > Send to Back.
Save as Template.
When you open that template to create a new document, the rulers will no longer show. Type command r if you want to toggle rulers on and off.
The Alignment Guides do not print.
This method will work for the first page only. Numbers does not have Section Master objects as in Pages.
Regards,
Ian.