Red Hat is very committed to putting the necessary infrastructure and organization in place around projects before they open source to make sure that the code isn't just available but can actually use community contributions. I don't have any inside info on this, but I wouldn't be surprised if OpenShift 4 is just waiting for that, or possibly to be in a stable enough state that the community can contribute.

Of course it's possible also that RH is keeping it closed for other reasons as well such as avoiding tipping their hand to competitors until their end goal is realized or something like that. I guess the point is I don't know, but given Red Hat's history of open sourcing even valuable acquisitions, I have faith that they will with OpenShift 4 as well.