I think Iansan will be a 4★

So far all 5★ natlanians (other than the Archon) have gotten their Story Quests as part of the Tribal Chronicles, meaning there can only be one 5★ tribesman per tribe. So either Iansan or Varesa will be that 5★, and the other a 4★. As Iansan has gotten some screentime (though sadly without sound in EN), and considering there is another 4★ coming in 5.5 (Dahlia or Ifa) - so the likelihood of introducing a 4★ Varesa via event drops significantly as the event is more likely to be about the other 4★ - I think Hoyo will put the newer character - Varesa - as the 5★.
(though I guess Dahlia could be a 5★ and, not being a natlanian, get a Story Quest, leaving the patch's event to a 4★ Varesa - this could be a win for everyone)