This post explains the hinges a bit. Apparently there was a defective run of hinges; if you have one, you can send Framework a video showing the bad hinge performance and get a free replacement.
The 3.3kg hinge is the same, although there is a more robust QC process in place to ensure no more defective hinges. The 4.0kg hinge is new and obviously stiffer (one-handed opening appears to be less practical with the new hinge).