I noticed that I had 60k feathers and I guess I'd thought I'd ask for any opinions on which three 4* characters to make 5*?
http://imgur.com/a/aWw5c
I was thinking of Lissa since she's been my main healer, but I have no idea really. I've just been trying to grind the 30k points in this event (they seem to have made it easier this time around compared to the last one).
You should check your units' boons/banes (or, to borrow Pokémon terms, IVs/nature). You'll feel bad spending 20000 feathers only to find out you've promoted a -Atk armour knight or a -Spd mage.
I would consider spending 66600 to upgrade Alfonse, Sharena, and Anna to 5*. They're always in the Arena bonus unit rotation, they're needed for the GHB quests, and they're all pretty solid units (Alfonse less so as you have Ryoma). The starter units all have neutral natures, so you don't risk accidentally promoting a bad one.
Otherwise, I think Effie, Cordelia, and Nino are my favourites out of your 4*s. You should also think about promoting some 3*s - as you already have a 5* Cecilia, you could try building towards a cavalry team by promoting Xander (who as a GHB unit also always has a neutral nature).
Healers don't really benefit much from promotion - they don't gain any new healing skills, and the increased stats don't matter if they're not getting into combat. Lissa isn't a particularly strong healer due to her low speed meaning she'll get doubled a lot.