During the Halo 4 Composer mission we can find the sixth Halo 4 Terminal.
The cinematic in the beginning of the mission shows us that Chief arrives in an area with a Halo Installation.
However, the artifact known as the Composer is actually inside the science station, located near the Halo ring. Cortana’s rampancy aggravates, and she crashes Johns ship into the station.
Halo 4 Composer starts at this point, when Master Chief speaks about evacuation plans with a doctor called Tillson.
We have to remember this name, because the sixth Halo 4 Terminal can only be found after we meet her in person.
The Master Chiefs pursuit of the Didact leads him to a UNSC science station orbiting Halo Installation 03.
RALLY POINT ALPHA
Ivanoff Station is overrun by the Didacts forces, who are intent on locating the Composer before the Master Chief.
RALLY POINT BRAVO
Dr. Tillsons revelation of the Composer results in a drastic course of action from the Chief.
Halo 4 Composer Terminal Location
The sixth Halo 4 Terminal can be found in the seventh mission known as Composer, only after we kill the two Hunters so we can activate the cinematic with Dr. Tillson.
After the cutscene, we follow the corridor towards the Composer, to begin the second part of the mission. If we wait a moment, Cortana will update the missions objective.
Then, while looking at the waypoint, we can see the writing ATRIUM on a building. The next Halo 4 Terminal is below that writing, inside a small cave formed by rocks.
We should note that there are actually three entrances to this small area, but from the angle mentioned above its easier to see the collectible.
The mission ends after the Didact gets the Composer, and then the final mission in Halo 4, Midnight, begins.