As I've yet to play Fallout 4, I figured I'd torture myself with this.
--Midwestern Brotherhood of Steel--
The Midwestern Brotherhood is now the largest branch of the Brotherhood of Steel, functionally governing large swaths of the Mississippi drainage basin. 20 years ago, the Midwestern Brotherhood expanded into the remains of the Gulf Commonwealth after rumors began to spread of an ever expanding forest. The Texas and East Coast Brotherhood all heard similar rumors and various divisions were moved to investigate, resulting in what, at the time, was celebrated as the reunion of much of the Brotherhood. However, after the initial incursion into the Overgrowth, it soon became clear that whatever technology that wrought this jungle onto the world was too difficult to reclaim. The Texas and East Coast Brotherhoods withdrew, but the Midwestern Brotherhood stayed behind, establishing a line of defense at the edge of the overgrowth and conducting a tactical nuclear barrage to burn its tendrils on the mainland. Those that stayed behind were charged with monitoring the growth of the mutant forest, and securing any technology they could find... but those patrolling the Florida Gulf know they're true mission. Banishment for the failure to keep the Brotherhood reunited, and loosing so many brothers and sisters to the overgrowth.
--The Brotherhood Fallen--
When the reunited Brotherhood of Steel attempted to venture into the overgrowth, they were met with horrors unfamiliar to any resident of the wasteland. Many brothers and sisters fell to the beasts, tribals, and general horrors of the overgrowth... but after the Brotherhood pulled out rumors began to spread that some of their fallen comrades had survived, only to be lost in the madness of the jungle. Eventually, some of these fallen brothers and sisters joined forces, pledging to wander the overgrowth in search of the ancient technology that birthed the mutant jungle. They're fanatical quest for the GECK at the heart of the overgrowth has made them battle hardened, but unpredictable. They never remove their armor, and few have working Fusion cores anymore. Caked in moss and rust, from afar they look as if the jungle itself is moving. Few who encounter them, mutant or human, friend or foe, live to tell the tale. As far as the Midwestern Brotherhood is concerned, they remain Fallen brothers and sisters.
After the destruction of Raven Rock, the remaining members of the Enclave fled the Capital Wasteland to a variety of fallback positions. In the 20 years since the events that drove them from the American capital, those cells have since reformed into a coherent force, based out of the old Charleston naval shipyard. This newest incarnation of the Enclave has taken to air and Sea to secure pre-war ports, and has had considerably more success than their predecessors. The Enclave has brought old shipyards back to life to build a new American nuclear navy. They've ventured to Florida to regroup with the last remaining cell to go quiet after Raven Rock, and rediscover the ancient legacy of the pre-war United States.
--The Vault Enclave--
When Raven Rock fell, a group of Enclave scientists and historians fled south, having been cut off from the rest of their cell. With no orders coming in, these scientists ventured south to the Florida coast, hoping to find the last remains of the original Enclave project organized by the pre-War American space program. Upon arriving over Florida, their Vertabirds came under fire, most likely by Brotherhood Fallen or Supermutants. Regardless, these scientists were left stranded in the overgrowth. Using ancient records from Vault Tec, they were able to find a nearby Vault that had opened itself to the Jungle 50 years ago, when Florida was more wasteland than overgrowth. Their numbers had dwindled since then, and it took very little effort for enclave scientists and a handful of soldiers to conquer their Vault. The survivors were ushered inside with all the Enclave tech they could carry, and the ancient door of Vault 121 was resealed. The war-weary scientists, historians, and soldiers, seemed defeated in their victory over this small tribe, and despite the initial hostilities, a kindship was born between Enclave and Vault-Dweller. In the 20 years since Vault 121 was resealed, its nuclear reactor has melted down, and its residents have been forced back up into the wasteland, to find a new home, and finish what the original Enclave started.
--The Pirates of Nassau--
When the bombs fell, a group of US naval officers in the gulf found themselves fortunate enough to be left as one of a handful of "missed targets." With no orders from Washington, these ships and their captains went their separate ways, choosing to retire in relatively unscathed Caribbean islands. In the two centuries that have followed, their decedents have taken up their old ships to defend their homes from the Brotherhood and Enclave that now threaten these waters. Taking the mantle of ancient Pirates of the 17th Century that ruled Nassau, these new brigands seek to free the Gulf from the rule of Brotherhood and Enclave.