I've had pistons that looked perfect on the top, smoked like that, but resulted in this:

https://www.miataturbo.net/attachmen...ine=1365862851

I've had others that looked perfect even after removing, but it was obvious the rings were no longer sealing.



your #4 looks bad; it looks like it's not seated evenly and rubbing in the 5 'o clock position of the photo.

on #3; looks like maybe some failure at the 10-11 o clock position on the edge of the piston...possibly melted a little.

I cant tell if it's just grim on the head pics, but all 4 look to have det on the squish area.