The 4th Brigade of the Operational Assignment Serhiy Mykhalchuk, also known as the 4th Rapid Reaction Brigade "Rubizh" is a brigade of the Ukrainian National Guard formed in 2016.

The National Guard of Ukraine is the Ukrainian national gendarmerie and internal military force. It is part of the Ministry of Internal Affairs, responsible for public security. Originally created as an agency under the direct control of the Verkhovna Rada on 4 November 1991, following Ukrainian independence, it was later disbanded and merged into the Internal Troops of Ukraine in 2000 by then-President Leonid Kuchma as part of a "cost-saving" scheme. Following the 2014 Revolution of Dignity, amidst the Russian intervention, the National Guard was re-established, and the Internal Troops were disbanded.