United States Naval Forces Southern Command:

United States Naval Forces Southern Command (USNAVSO) was established on February 17, 2000 and headquartered at Naval Station Roosevelt Roads, Puerto Rico, later relocated to Naval Station Mayport, Florida in early 2004. USNAVSO serves as a main link between the U.S. Navy and the navies of South America, Central America, and the Caribbean, and is at the forefront of U.S. engagement in the Western Hemisphere.

The command consolidates functions previously conducted by the former Western Hemisphere Group, Mayport, Florida and former South Atlantic Force, U.S. Atlantic Fleet (SOLANT), Naval Station Roosevelt Roads, Puerto Rico.  USNAVSO was built around the core of SOLANT, which was originally established on September 3, 1942 eventually headquartered at Roosevelt Roads, Puerto Rico in 1973.  SOLANT had participated in annual "UNITAS" deployments around South America since 1960 and routinely deployed to Africa for the West African Training Cruise since 1980. 

 In addition to UNITAS, USNAVSO participates in Navy and joint exercises, counter-illicit trafficking operations, port visits, humanitarian missions, disaster relief and protocol events. USNAVSO exercises military command and control of all assigned/allocated ships and units, and serves as the Navy component command to U.S. Southern Command (USSOUTHCOM) with respect to naval matters in the region.

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U.S. Fourth Fleet:

U.S. Fourth Fleet employs maritime forces in cooperative maritime security operations in order to maintain access, enhance interoperability, and build enduring partnerships that foster regional security in the USSOUTHCOM AOR.
On April 24, 2008, Chief of Naval Operations (CNO) Admiral Gary Roughead announced the reestablishment of U.S. Fourth Fleet. He officially reestablished U.S. Fourth Fleet effective May 5, 2008 with a formal ceremony held at Naval Station Mayport on July 12, 2008.

The original FOURTH Fleet was a major U.S. Navy command in the South Atlantic Ocean during World War II. First established on March 15, 1943 to protect the U.S. against raiders, blockade runners and enemy submarines until renamed Task Force 27 on April 15, 1945 then dissolved in the fall of the same year.
On January 10, 1946, the FOURTH (Reserve) Fleet was reactivated to initiate the Reserve Training Program in support of Emergency Plan White until January 1, 1947 when, in accordance with NavOp #14, it was dissolved. On January 1, 1947, VADM D. E. Barbey hauled down his flag as Commander FOURTH Fleet.

Commander, Service Force Atlantic Fleet (COMSERVLANT) assumed the duties of Commander, FOURTH Fleet for the preparation and implementation of Emergency Plan White.

U.S. Fourth Fleet headquarters is co-located with U.S. Naval Forces Southern Command (USNAVSO) at Mayport, Fla. U.S. Fourth Fleet Commander is also Commander, U.S. Naval Forces Southern Command (COMUSNAVSO). U.S. Fourth Fleet is responsible for U.S. Navy ships, aircraft and submarines operating in the USSOUTHCOM area of responsibilities (AOR), which encompasses the Caribbean, Central and South America and surrounding waters.

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Summary of 4th Fleet history:

11 Jan 1941, Rear Admiral Ingram hoists Flag in Memphis as ComCruDiv 2 and CTF 3

10 Feb 1942, Ingram promoted to Vice Admiral and TF 3 was changed to TF 23 (Separate command report states TF changed occurred 6 Mar 42)

16 Sep 1942, Vice Admiral Ingram designated Commander South Atlantic Force (COMSOLANT) (SOLANT designation occurred 3 Sep 42)

15 Mar 1943, South Atlantic Force (SOLANT) becomes FOURTH Fleet; Admiral Ingram retained COMSOLANT

15 Apr 1945, FOURTH Fleet becomes TF 27 of Atlantic Fleet; Admiral Munroe retained COMSOLANT

Fall of 1945, FOURTH Fleet and SOLANT was fully dissolved and all records destroyed

10 Jan 1946, Rear Admiral Cooley, ComBatDiv 4 and ComBatCruLant, assumed temporary duty as Commander, FOURTH Fleet (Essentially FOURTH Reserve Fleet responsible for Emergency Plan White readiness for all of 1946)

23 Feb 1946, Vice Admiral McMorris assumed command of FOURTH Fleet

24 Sep 1946, VADM Barbey assumed command of FOURTH Fleet

1 Jan 1947, FOURTH Fleet dissolved IAW NavOp #14, Barbey hauled down his flag

The history between Jan 1947 points to reserve fleet duties to be assumed by ComServLant until around Feb 1950 when Second Fleet assumed duties of the South American coast. ComSoLant was reestablished 1 Jun 1958 and FOURTH Fleet 5 May 2008.