What do you need more? In a vacuum, I’d probably promote Robin, as she seems to have the best upgrades going from 3* to 4*, but if you’re flush with green tome users, why bother? I’d consider promoting my 3* Nino if I didn’t have a 5* Julia and 4* Cecilia filling those needs for me.
I’m looking at a 3* MRobin, 2* Olivia, or 2* Sharena as my most likely candidates for promotion … once I get them to level 20. I think Olivia would be the most worthwhile, since I don’t have a dancer or singer and my other red troops are mediocre at best, but that’s a long road …