Hi Alan:
Not to disappoint, but the nasal openings from inside are limited by bone all the way around, including the septum in the center. If you look at a skull you will see the thin bone in the center of the nasal opening. The opening will not change much. The way further in is by whatever can be done with the tongue -- twisting, pushing, stretching, etc.
But it is not that big a deal really. After 20 years of playing with kechari, I stay mostly around the septum -- the "secret spot," and use stage 4 (into the nostrils) sometimes for alternate nostril breathing from the inside when doing chin pump. It can be used that way with any form of pranayama, including spinal breathing. But be careful not to overdo it. Once ecstatic conductivity is coming up, less can be more.
You have the angles and everything right for stage 4. It will go as far as it can go. Up is where there is the most running room in my case. Some don't do stage 4 at all due to anatomy. I don't think it makes a big difference. Many reach higher levels of ecstatic conductivity with stage 1 alone. There are few absolutes in this world, and the level of kechari we achieve is not one of them. (Though getting above the soft palate is obviously a pretty big step.)
The guru is in you.