In the Lore, it seem we can split ourselves into 4?

[Rocl]

It's common courtesy to spoiler tag things for a couple weeks after release, or at the very least mention that you're going to spoil early on in the post.

That said, I encourage you to read the Duty Finder descriptions of, like, every dungeon and read carefully. Every trial and dungeon this patch has done a fantastic job of answering this question: you don't "split into 4."

To wit, from the Mettle

Each must do this by themselves, of course, and so you shall...though if by pure coincidence some of your fellow adventurers happened to have made the long journey to the Far East, and simply insisted on accompanying you...surely no one would be any the wiser?

Canonically, some of our adventurer friends might've insisted on helping us in the Mettle. You might not have any friends to help you too, if you wait a year or two and unsync it.