For now, E-4 is not being propagated due to various ripening issues. It is known to have some issues over the years in addition to being difficult to k ow the right time to harvest.
These mangoes are observed for many years before final decisions are made. As you can see, some are still pending/up in the air as to being propagated. Who knows, 3 years from now something could change and warrant its propagation. There are many factors that go into the decision, not just taste. For instance, on taste alone Karen Michelle would be propagated however it is a consistent shy bearer. This reason alone will prohibit the propagation of it.
If I had to guess, i would say Kathy and Cotton Candy are the next possible releases on the horizon.