According to a Tweet published by the "Ukraine Weapon Tracker" on May 14, 2022, approximately 7,000 small arms, including M4A1 5.56mm caliber assault rifles, were recently donated by the United States as military aid for Ukraine and now the M4A1 is in service with the Ukrainian army.
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A U.S. M4A1 5.566 caliber assault rifle was unpacked by a Ukrainian soldier. (Picture source Twitter account Ukraine Weapons Tracker)

Since January 2021, the United States has invested more than $4.5 billion in security assistance to demonstrate our enduring and steadfast commitment to Ukraine’s sovereignty and territorial integrity. Since 2014, the United States has provided more than $6.5 billion in security assistance for training and equipment to help Ukraine preserve its territorial integrity, secure its borders, and improve interoperability with NATO.

Citing information released by the United States Department of Defense, in terms of small arms and weapons, United States security assistance committed to Ukraine includes 1,400 Stinger man-portable air defense missile systems, 5,500 Javelin anti-tank guided missiles, 14,000 other anti-armor weapons, 7,000 small arms, and over 50,000,000 rounds of ammunition.

The M4 series 5.56 mm Carbine is a compact version of the M16A2 Rifle and is equipped with a collapsible stock, an adaptor rail system, and backup iron sight. The M4A1 is fully automatic and incorporates an ambidextrous fire control selector. The weapon can be mounted with the M203A2 Grenade Launcher, M320A1 Grenade Launcher, or the M26 Modular Accessory Shotgun System.

The M4/M4A1 Carbine enables individuals and small units to engage targets with accurate, lethal, direct fire. The M320 series grenade launcher enables Soldiers and small units the ability to engage targets with lethal grenade fire. The M4A1 has a weight of around 3.5 kg and a maximum firing range of 500 m. It uses a standard 30 rounds magazine.