Time Line

He's still King in this film, but Mathayus is doubling his time between his building kingdom and personal mercenary missions. This movie is probably the most "filler" as it's a real stand-alone comedy, whereas Rise of a Warrior is a further origin prequel and Battle for Redemption shows the character rebuilding his fallen kingdom. The order goes (EDITED to how I now view it):

The Scorpion King 2: Rise of a Warrior
The Scorpion King
The Scorpion King 4: Quest for Power
The Scorpion King 3: Battle for Redemption
(The Mummy Returns' prologue)

The Mummy
The Mummy Returns
The Mummy: The Animated Series
The Mummy: Tomb of the Dragon Emperor

Superman & The [Red-Blue] Blur = The Man of Steel

Rewatching the movies, honestly Quest for Power can be set either after or before Battle for Redemption, there seems to be more signs Mathayus is just a mercenary than a resumed King and the tales of his reign are even "campfire stories". Really the only thing off about the film if setting it before Battle for Redemption is Mathayus is in much more of a fun and humorous mood, whereas he's far more in a brooding state within the third film. I'm gonna stand corrected with my own point of view of the timeline, I personally view Quest for Power before Battle for Redemption. The Scorpion King 3 is at a later enough point that Cobra heard of Mathayus' war with Memnon when she was a younger girl, while we know only about 10 years pass between the original Scorpion King and The Mummy Returns' prologue from behind-the-scenes interview, then Battle for Redemption shows the power-restored King more battle-hardened and subtly militiant now willing to trade his life for his nobility.

Superman & The [Red-Blue] Blur = The Man of Steel