4th member of Flier Emblem team?

If you would rather just make your current 3 more powerful, Camilla is your best bet. Her axe plus a ward fliers and drive atk ss can give any fliers within 2 spaces +10 atk and +7 spd. Using hone fliers, it can be 12/9. On the other hand, Cherche with a brave axe and death blow can deal some crazy damage, and desperation plus brash assault ss can let her quad if her hp is low enough. However, yours doesn't have the best ivs. Shigure seems a little better stat wise than Cordelia, so if you want to use one of them, use Shigure. Personally, I would go with Camilla because she is a great flier buffer, and you have a blade tome on your team who could use the buffs. (So the hone fliers would go to regular Camilla to buff summer Camilla)

I really appreciate that you've thought this through unlike half the questions we get here which are just like "what should I do with ____ kthxbai". I'm content to answer maybe a few of those, but the proportion of those we get is really more than I enjoy. So thanks

The double HF+FF idea is interesting. I'd actually use a dancer if that's the setup you're going for. I consider goads the way to really maximize the team's potential generally, but you have a team with two enemy phase units so I think your focus on visible buffs is not bad.

Of the choices you offered, I would probably put the hone on Myrrh, actually, especially if your fourth member will not be a dancer. I think Myrrh needs the hone the least unless there is a dancer.

And if you're not going to use a dancer, let the last member be a non-blue. Your principal player phase unit is blue; it is preferable that your second player phase unit have no fear of high-res greens and maybe have some def. Cherche fits that description, but you could also look into L.Ryoma if you pull at all on legendary banners.