Brave Defenders of Humanity

Rear Admiral Augustine Terrell, head of SECCOM for all UNSC forces in his AO, is responsible for the defense of the remaining worlds of his sector.

Rear Admiral of the UNSC Navy and commander of the Fourth Sector

UNSC forces in the sector fit into two categories: the organic defense groups assigned to individual systems and regions, and expeditionary units assigned to the sector from the UNSC's main forces. Defense ships are easily recognized by the green markings on their hulls, where as expeditionary ships lack these. In addition, the United Nations Colonial Star Guard (UNCSG) maintains numerous smaller vessels and stations in the region, in addition to several frigates and patrol craft to assist the Navy.
The fleet is mostly concentrated around strategic assets and systems, like New Constantinople, Port Zanzibar, and along the Draco Spur. Systems closest to what is considered "Covenant space," former human systems gradually lost over the war, host small squadrons of ships for defense. Many systems can also boast defense stations and numerous planet-based fighter and bomber squadrons for organic defense.
Most of the fleet is of lighter tonnage, with the Paris-class heavy frigate being by far the most common class. The Paris, as the most-powerful of the frigate classes, is employed almost exclusively to leverage against the small numbers of larger vessels available.

UNSC heavy frigates in "Fleet Grey" are a common sight in the region, often in lieu of larger vessels

Destroyers also appear in some numbers, typically around important task forces and locations.

Cruiser hulls are exclusively Marathons, with all lighter cruisers lost to the Covenant in the dark, early years of the war and not replaced. These are the flagships and backbones of the sector's task groups.

The heavy-hitters of the UNSC Navy, Marathons are the Navy's best defense against high-tonnage Covenant vessels that appear in the Sector

Two carriers are currently directly assigned to SECCOM, an Epoch-class heavy carrier and a Cyclops-class light carrier, a rare subclass of the Marathon cruiser hull. Both of these ships, and their escorts, function as fire-brigades of sorts, responding to the most dire threats against the Sector.

The only heavy carrier in the region, she is also the largest ship Reach is currently willing to assign to the area

Sacrificing much of her missile throw-weight and cargo space, the Cyclops sub-class boasts greatly increased hangar capacity and more robust communications and flight control

One of the first deployments of the UNSC's energy field technology, greatly furthered by captured Covenant material. This vessel has energy fields as opposed to bay doors for her fighter recovery decks. An armored bay door still protects the launch tube at fore due to lingering fears of gaussing from the nearby MACs

Additional forces beyond the Navy defend Fourth Sector's space, ranging from special forces to second-line status.

Unbeknownst to the UNSC proper, ONI maintains several highly modified frigates for missions beyond the capabilities of the Prowler Corps

Though currently assisting in the survey of the artifact-rich Zeta Prime system, there is little doubt the CINCONI would deploy these frigates if it served ONI's purposes; even if it meant operating in the open with UNSC forces

As mentioned, the United Nations Colonial Star Guard (UNCSG) is also present in defense of the region. The organization is a holdover of the CMA absorbed by the UNSC during the Insurrection, and is notable for having a dedicated law-enforcement mandate as well as military capability. Several patrol groups of frigates, assigned to the area since before the war began, are engaged in escorting convoys to frontline systems, standing vigil over important orbital and deep space installations, managing Insurrection flare-ups, and protecting against Covenant raiders. This in addition to assisting UEG authorities with refugee management, customs duties, and disaster relief.

Hard to mistake with Navy vessels, UNCSG frigates used to running-down smugglers and Insurrectionists now dodge plasma fire while assisting the UNSC Navy. They are the heaviest assets available to the organization

Of note, FLEETCOM on Reach has begun to view the large facilities available on Port Zanzibar favorably as a base for long-range raiding fleets, the kind typically centered around Orion-class carriers and their support groups. These are unlikely to be on-hand for sector fleet operations, but could still represent powerful reinforcements if the high command decides to proceed...

On a real-world note, very sad news today in that Spartan Games, the producer of my favorite Halo-themed starship combat game, as well as Dystopian Wars and Firestorm Armada, has closed down today very suddenly. Prayers to the owner of the company, who is not in good health according to the Facebook community. Hopefully, another company picks up where Spartan leaves off...