I have a long, long overdue write-up about T4 Souls coming up. But the short version is:
First T4 Soul should be Medea of your strongest element.
Your second T4 Soul should be Yori, and preferably all element Lances (or at least all your strong elements).
Only after those are done should you be looking at other Souls, IMO. Explanations coming up once I bother writing the thing.
As for Charlemagne specifically, she is very underwhelming right now. She'll be decent for a T4 Soul once buffs hit, but we don't know when that'll be (June 2021 at the latest, so a long, long time away).