Should I regroup Sophia?

No, you shouldn't.  maxed-Sophia is many-times over more valuable than maxed Victoria.


Assuming you haven't finished all the campaigns, you should get Sophia to 5-stars navy (possibly in Move as well) and maybe Isabela too before bringing in Victoria.

You are in the same situation i was. I only had 4 slots and i decided to take in victoria as my forth general. Don't regroup sophia because she is the second best princess imo after Lan. i Just finished all the sns campaigns with 5 stars and Victoria is a must. She is more useful as navy general than infantry because there are a few navy missions in sns which  are quite difficult. My victoria only has 3 infantry stars but i gave her 2 6+ infantry attack items so she still deals lots of damage. Make sure to s/l with her so her defense and assault art triggers. I recently wrote a guide on how to trigger defense art so you might want to check it out. 

Before you take victoria in, Give sophia 5 naval stars like i did. This is up to you but you might want to give sophia some movement stars aswell so she can move 4 hexes on any artillery. She will require 5 movement stars, a warhorse and a 2+ movement item. It's expensive but moving 4 hexes makes a big difference if you want 5 stars in campaigns. 

Sophia > isabella if you use her right. Lan is the best, Victoria is very good if u sl/ to trigger assault and defense art.