Forward Operating Base 4 (also known as FOB 4) is a former Military Assistance Command, Vietnam – Studies and Observations Group (MACV-SOG) Command and Control North base.

It was near the Marble Mountains southeast of Da Nang, Vietnam.

Seabees from NMCB 12 built sea huts on the base in 1968.

[2] On the night of 22–23 August 1968, as part of their Phase III Offensive, a company from the Viet Cong (VC) R20 Battalion and a sapper platoon infiltrated the base, killing 17 Special Forces soldiers (their largest one-day loss of the war) and wounding another 125 allied soldiers.

[3] The base is abandoned and the area now consists of sand dunes with some commercial and residential development.