should I keep working in the A4 sketchbook or move on?

[Olooriel]

Depending on what kind of work you do in them, you could maybe either keep the A4 one for a specific theme you're more motivated for, or use it for a specific type of practice/sketch that makes it easy to fill pages with lots of smaller things, like thumbnail studies, or gestures - large pages don't necessarily mean you have to draw big.