FR4 is a high-density fiber material that feels given between an aluminum plate and PC. It is solid with a certain flexibility and also solid with hardness, which enables you to get a balanced typing experience.
What's in the box
It's lighter and gives much more thock than their standard steel plates. But the plate is too thick for the switches to clip in. I'm currently using gateron oil kings and they don't clip into the plate at all. They clipped into the standard steel plate just fine, and on a brass plate they would clip in with no problems. But the fr4 plate is slightly too thick compared to either of the other plates. If I were to pull off the keycap the switch would come along with it. In the end the keyboard still works and the switches are held in with enough tension to never be an actual problem. But I would prefer the fr4 plates to allow switches to clip into it.