It would be best if you work with your engine builder to source the pistons and rings.
Bore as small as possible, and we find 4.0" bore and larger will get you a proper piston and ring package.
Also, you need to decide on the compression ratio you want so the dish in the piston is compatible with the block and heads.
Over the many years and builds of 4.1 turbo engines, we have a "selection" of custom pistons from the 2 major manufacturers we use.
You would not be happy after spending big bucks to find out pistons/rings would not work out for your block? We completely machine and prep the block before we order pistons.
We had a customer order a complete rotating assembly for a 4.1 build, and it would not work in his 4.1 block. Fortunately we had a few other 4.1 builds going through the shop at the time so we were able to work it out for him.