Originally there were going to be three Mistborn eras, but then the Wax and Wayne books just kinda happened. So there will be four. The two we have, one at a ~1980s tech level, and one with a sci fi future tech level.
Allow me to be the first to say here: Read And Find Out.
Specifically, the most info we have is from Mistborn Secret History. But even with that, there's still a lot of questions. Most of what we know is pieced together from a line here or there, or questions people have asked Sanderson. Also a...