Messier 4 (also known as M4 or NGC 6121) is a globular cluster in the constellation Scorpius. It was discovered by Philippe Loys de Chéseaux in 1746 and catalogued by Charles Messier in 1764.

M4 was the first globular cluster in which individual stars were resolved. Just visible to the naked eye, M4 is conspicuous in even the smallest of telescopes as a fuzzy ball of light. It is also the one of the easiest globular clusters to find with it only being 1.3 degrees west of the bright star Antares with both objects being visible in a wide field telescope. Modestly sized telescopes will begin to resolve individual stars of which the brightest in M4 are of apparent magnitude 10.8.

At the distance of 7,200 light-years that has been determined for M4 it is perhaps the closest globular cluster to our Solar system. It is also a rather loosely concentrated cluster of class IX. M4 appears about the same size as the Moon on the sky which, given its distance, yield a spatial dimension of some 75 light years across. At least 43 variable stars have been observed in this cluster.