Olivia

A 4*+10 unit is better than an unmerged 5* unit.

4*+10 has higher BST and scores better. That said 5* merging Olivia isn't a bad investment by any means.

If you have her highly merged already, stick to 4*+10. But if you are considering this right now and have not yet acted upon it, 5* max merging is always better if you want to use her long term.