I just went over The River of Cradles to find some ties to notes I made for a character (a shaman, of course 😜 b. 1592) I played in a campaign with Magister Ludi back in '98. Reading the timeline it hit me: my character was part and parcel of history. And that history is still ongoing; so much so, that it begs the interesting question of how that character fares in the year 1627. Enter Conversion Guide rules.
Maybe that's the secret to the fascinating attraction of the World of Glorantha: it's a living world, because it continues to live on in the joint imagination of all of us. That's the very definition of Myth, by the way...
Back to OT: What's out there so far for the 4th edition of RQ (and the world of Glorantha) is a good headstart on the background. Of course, more is always welcome.
As a GM new to the material I'd focus on the status quo, i.e., ordinary people dealing with their everyday problems: like, where their next meal comes from, or how their neighbouring clan comes raiding cattle again; who suddenly must face -- and eventually overcome -- challenges that are out of the ordinary. That's how heroes arise. No need to know (as a player OR GM) what King of Sartar currently holds sway (if any), or what happened in the year 1616 elsewhere; that background (which may serve either as a precaution or mould of possible behavior) is always there to be gleaned, as the protagonists go along making up their own story and either grow (or devolve) because of it. The important thing to remember is that they eventually may/must sense they live and act in a context bigger than themselves, and have the choice to act accordingly (or not). A heroes journey is never easy, that's why it makes for interesting stories.