Type I Metroid games have a classical gamestyle with classical items, with a focus on speedrunning and sequence breaking. Type II Metroid games are more linear, less speedrun-friendly, and far more story driven. They often feature different items and styles as compared to previous releases, and are far more experimental.

This is pretty much on the money. One thing though, I think "focus on speedrunning and sequence breaking" isn't exactly true, as Nintendo doesn't expect most players to get that deeply into the game that they specifically try to do those things. Instead, their compact and more open ended designs specifically allow for speedrunning and sequence breaking.

I think the reason that Type II style happened was a need to make the games longer, getting the most out of the limited assets of a gameboy game or adding padding to an expensive 3D title. I only time I really think this worked was Samus Returns, which broke the game into interesting chunks and put a lot of thought into keeping the combat fresh. I don't remember if you had an opinion on that game, I love SR.

In any case I think Nintendo is scared to release short games and has only recently gotten over it, though we'll see how Link's Awakening does.

Maybe it will be that Metroid Prime 4 will just be Metroid Prime 1 Two: Electric Boogaloo, because that's a really successful formula. Who knows?

I have no idea what the original version of MP4 was, but now that they've rebooted it the way they have I'm pretty sure that's what we're getting. It's not the same team that made the first Prime games working on it, most of them left after 3, but I can't imagine that Retro Studios would be involved if it would be some huge departure. I'm very pumped though, Tropical Freeze was so fucking good.