Why are my new A4 documents [*******] HUGE??

To add to what was said above by servitor.

You need to understand that in Photoshop, 100% zoom means display 1 image pixel for 1 screen pixel. That is it. It has no connection to any physical document size.

To see the document at the proposed print size (in your case A4) , then use  View - Print Size. Photoshop will set a zoom level to display the same size on screen as it will print. However for that to display correctly you need to set your screen resolution accurately in Preferences - Units &Rulers so that Photoshop knows how many screen pixels will represent one inch on your monitor.

Dave

Photoshop is creating the A4 document at a resolution of 300 pixels per inch (PPI). An LCD Screen displays between 67 and 130 PPI.  100% display for the A4 document is displaying a rectangle 2480 x 3508 pixels. The printer turns it into 210 x 297 mm.

A pixel is a picture element. The smallest element the eye can see on the screen . Zoom in 600% to the photo on your computer screen and you’ll see them: rows and rows of tiny little squares. These are also the smallest addressable unit of a digital image.