Don't know if someone has done this already, but I dove back a couple pages and didn't see it:
Since feathers are a major limiting factor in upgrades and the jump from 4* to 5* takes a ton, I went through the drop list to iron out which 4* you should upgrade vs. which ones you shouldn't because they have a natural 5* drop.
I'd recommend not spending the feathers to upgrade these folks from 4* to 5*:
Red: Ogma, Cain, Corrin (male), Chrom, Caeda, Marth, Roy, Tiki, Tharja, Lilina
Blue: Abel, Effie, Catria, Peri, Nowi
Green: Camilla, Sheena, Hawkeye, Raven, Fae, Merric
Grey: Jeorge, Kagero, Jakob, Sakura, Maria
Any other 4* that drops doesn't have a higher version (such as Eliwood or Azama, for example), so I would make those a priority to take up to 5* if you have the feathers.
All of those "other" 4* do also have a 3* version so don't bother updating a 3* to a 4* unless you love them and want them boosted ASAP. If you can wait for drops, wait for drops.