Best 4* +10 units?

Cherche far and away is one of the most spectacular 4* +10 merges, as you can find an ideal boon/bane and she's readily available at 3* and 4*.
Another good choice is Olivia, as she greatly appreciates the increase in her stat line.

Cherche, Olivia, and Sophia are all good at 4*+10

Nino. I've got her at +4 right now, Soleil also will be up there now that she's available.

Cecilia is great aswell, i was building a roderick with lnd which was great at +9 with 50 atk and 45 spd.

Nino 3-4/ Nowi 4/ Cordelia 4/ Olivia3-4 / Cherche 3-4/ Leon 4/ Tailtiu 4 and now new Soleil 4 are the top one i would say in use and stats wise.
Nino Olivia and Cherche are the easiest to get fast 4star+10 thx to be 3 star available.

Nino and Cherche imo are the two best units for a 4+10 merge. Here's my 4*+10 Nino, she is a definite powerhouse. She is +spd, -def.

Some additional units available at 3 stars that can be pretty good are Robin-M, Hana, A!Tiki, Shanna, and Catria.

Cecilia is the best, no doubt. Olivia and Tiki are second as both are common and Olivia doesn't require her 5 star weapon nor gets any skill. Those 3 are the best with Cherche pretty close imo, you can't go for rare 4 star units like Cordelia imo.

I'll throw in adult Tiki since a lot of people here aren't really mentioning her. With the new lightning breath + it makes her way more viable and with her insane defence and how prevalent brave Lyn is right now she is pretty solid to have, also she is capable of killing Julia or Deirdre on a counter in a lot of situations because of her high atk. Here are the stats from mine, I believe it's +atk -res

Cecilia, OLIVIA, Nino, and Adult Tiki! Cecilia is the best Reinhardt and Brave Lyn counter!