Yosho ends on a cliff hanger. Washu ends abruptly... Airi's and Minaho's backstory is only ever lightly touched on. Widow is talked about like you should know her because she murdered everyone important on Airi but she only gets a few lines. Seto is left a total mystery, we don't ever get the story of the mysterious girl in the picture in any of the stories so far or her name although like quiz panta, seto's alter ego, she is also a widow in seto's charge and we know very little about seiniwa and what happened to mikumo or mikami or mihoshi....
so ... the conclusion to me is obvious.
There was meant to be a second set of True tenchi novels and they haven't come out because OVA 4 was meant to be apart of OVA 3 which was cut short and these are supposed to come after them.
It wouldnt be the first time a cover has been a spoiler for something long into the future... for example...
Oh look its ova 4.... in 2003... when it came out in 2017....
and what about the notan projects... where the true tenchi novels were pictured on the cover next to ova 3... years before any of them ever came out.
Kajishima has said he already has the whole story thought up and planned out.... he just isn't able to release it at the pace he'd like.
so it seems obvious to me that their are 3 more novels that are counterparts to the original 3;
Jurai Yosho Washu -> Seiniwa, Airi, Seto