can someone help me with a question about cecillia

A merged 4* will be as strong as a 5* if at +5. Any higher will be a stronger unit than a non-merged 5*. So a +7 will be fine.

However, you need at least one of the fodder Cecilias to be 5* if you want to access Gronnraven+.

A 5+ unit will boost your arena score immensely. The best of the best use all 5 star + 10 units. I'm in tier 20 top 2000 rn and when I do the arena, the lowest I try to go is 5* + 4 or 5, which hovers between Ike and Eirika.

Save your feathers, if you're able to merge to +7 as is the difference is marginal, and she'll be even slightly stronger than a base 5 star iirc