There are two way to estimate what a writer/author will do in his next work:
There is what and how he wrote in past. in this case what is obviously Tenchi Ryo, and the second is what he wrote in OVA3.
OVA3 was a prequel of Tenchi GXP, Therefore I will suggest the OVA4 might be prequel of War on Geminar
As to that Kajishima Sensei is currently writing, we have the two Doujinshi line as possible candidates,. The IF doujinshis state clearly they were intended to become a new anime series and could easily become one, but Kamidaki doujinshi dealing with Kenshi would be more in the prequel track.
Given this, I think there a greater chance that OVA4, if it is produce, will center on Kenshi’s early life.
