M-4 is descended from Keitt. M-4 is Mid-to-Late Season, but not Very Late Season.
The fruit size is small. The number of fruits produced is high. The original tree has been an alternate bearer.
No grafted trees of it have been evaluated. Same with 'Cotton Candy'(also from 'Keitt' and mid-to-late season). These two varieties were released due to high connoisseur and staff insistence, despite not having yet undergone a grafted-trees-field-trial planting. This insistence was because of supreme deliciousness, to pretty much everyone who tried them, and clean appearance, though not at all colorful.