The four languages are Sabaean, Minaean, Qatabanian, and Hadramautic. Together with Ethiopian Semitic languages (such as the contemporary Ge'ez language) and the Modern South Arabian languages (not descended from Old South Arabian but from a sister language), they form the western branch of the South Semitic languages.
Old South Arabian had its own writing system, the South Arabian alphabet, concurrently used for Ge'ez in the Kingdom of D`mt, ultimately sharing a common origin with the other Semitic abjads, the Proto-Sinaitic alphabet.
The arrival of Islam virtually disintegrated Old South Arabian, as Classical Arabic became the lingua franca of the region. Today, Old South Arabian exists in a few ancient texts and inscriptions. It has contributed to the local Arabic dialects of the region in much the same way that Coptic has contributed to the Egyptian dialect of Arabic.