I recently came across two Circa notebooks I bought several years ago that are 4 x 6, approx. They called it "Memo" size. I liked the size, but it wasn't carried long, apparently.

I had gone on a quest to find refills that would fit, yet be fountain pen friendly.
The closest I found, apparently was an Ampad "Pastels" notebook. 170 ruled sheets, 4" x 6", 5 assorted pastel colors. #19476
The paper fits the notebook well. The biggest drawback is that there is an almost 1" margin on the top of the page. And it is "wide ruled", but that's good if you write large, I guess.

That's a long way to say "I wish Levenger would make this size available again."