4.2 was really created for High School age athletes who have strong stunting skills, but lack some of the higher level tumbling. I would be surprised to see it go junior, but I guess anything is possible.
I feel like 4.2 was created for senior athletes mainly because as you get older tumbling gets difficult (you lose that youthful fearlessness) and many athletes who start later get stuck in their progression once they get a backhandspring/ walkover combination skills. Senior athletes are also more likely to be more capable of handling more difficult stunts but if they are stuck in the senior 2 division in their whole life any talent they have in other areas of cheer would be squashed
Like the previous posters said above, stunting is normally easier to get the older you are. I feel like Senior 4.2 is big because it let's those kids who started later in life with cheer to show off higher level stunts, without worrying about tumbling. There really isn't a need for junior 4.2, because a 9 year old still has 9 years to learn higher level tumbling skills and 10 and up are allowed on senior teams.