Do I cut the brine in half if I would like to use 4 pcs instead of 8?

Yes, you just need to cover the chicken pieces.

Absolutely.

The point is to submerge the chicken pieces in the brine. Having more brine doesn't do anything. Also, making extra brine and freezing some of it doesn't make sense either since brine is so cheap & quick to make and it unnecessarily takes up a lot of freezer space.