I am desperately looking for a large notebook (A5-A4) with 4mm squared paper; do you have any advice for me?
This took some hunting and I could only find one printed option that featured the coveted 4mm grid paper It’s the Miquelrius leather-look journals. They are available in 100-, 200-, and 300-sheet books with black, blue or red covers for $9.99 to $14.99. The paper quality is decent. I used a Miquelrius book for ink testing for some time early in my blog career before switching to Rhodia paper which is a bit more hardy.
There are more images of the whole Miquelrius Grid journal I used available on Flickr.
Another option for grid paper would be to print your own paper. Paper Snake offers printable graph in a variety of quadrille and graph paper sizes in metric and imperial sizes including 4mm.