You can buy couplers with 8mm one end and 10mm the other (10mm common side for a 1605/10 ballscrew end.

Or some places offer custom machining (e.g.BST Automation) for the ballscrew.

8x4 is quite big so rack and pinion often used on the long axis. I think some people have used 2005/10 ballscrews but can’t be sure so wait to see what other have used.

Depends on what you are using it for and what speed you need and therefore which pitch.