Complex sentence: Among the most prominent definers of the Glasgow School loose collective were The Four: the painter and glass artist Margaret MacDonald, acclaimed architect Charles Rennie Mackintosh (MacDonald's husband), MacDonald's sister Frances, and Herbert MacNair.
Split into simpler sentences: Among the most prominent definers of the Glasgow School collective were The Four. They were the painter and glass artist Margaret MacDonald, acclaimed architect Charles Rennie Mackintosh (MacDonald's husband), MacDonald's sister Frances and Herbert MacNair.