Is it better to merge 6 x 4* Cherche or upgrade one single Cherche to a 5* ?

Do you plan on using her? If you want to use her in flier emblem and want a powerful unit, I’d say make all of them 5* and merge them. But if you don’t care that much, merging 6 at 4* would make her a 4*+5, which would put her about on par with a 5* unmerged Cherche.

^ what he said

Merge a couple of ninos in for speed, and some nowi for additional defense.

Thank me l8r

A +10 4* has around the same stats as a +3 or +4 5*. If you feel they're taking up too much room, by all means go up to +10.
I mean, unless you don't plan on going up that high. If that's the case, either save her or make her a 5*.