The ending of The Expanse season 4 explained – Looper

One of the main storylines of season 4 is the conflict on the newly discovered planet called Ilus, or New Terra. A group of Belter refugees (people who live a hardscrabble life in our solar system's asteroid belt) has colonized the seemingly hospitable planet. However, they are quickly set upon by a UN-backed research group called Royal Charter Energy (RCE) who wants to drive them from their new home.

While Ilus wasn't inhabited when the Belters and RCE arrived, it clearly was at one time. James Holden (Steven Strait) is sent to the planet to monitor the situation. But when he arrives, alien structures that appear to be powered by the presence of the protomolecule spring to life, threatening the Belter colonists and RCE crew alike. Holden has some protomolecule traces in him, which is what allows him to see the specter of Joe Miller (Thomas Jane), who was subsumed into the powerful blue goo earlier in the series. The specter is the protomolecule coming to Holden in Miller's form, but eventually what's left of Miller's consciousness gains enough control to act independently, and he is able to help Holden put an end to the chaos on Ilus.

Miller leads Holden to an ancient alien device (no, not those Ancient Aliens) that is leftover from an intergalactic war long, long ago. With the help of Elvi Okoye (Lyndie Greenwood), a geologist with Royal Charter Energy, Holden delivers Miller to the device, which shuts down the structures on Ilus. It's a victory for humankind on the frontiers of space, but it's unlikely to be the last alien threat the species faces.

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The ending of The Expanse season 4 explained - Looper