By the end of S04E11 - Queen Takes Knight episode, Poison Ivy consumes all the "mystical" potions since she is done playing "nice". The resulting drinking of potions changes her face and gives her more powers...
Based on the co-exec producer's comments on recasting Maggie Geha:
"The change was all based on the idea that she's continuing to evolve to become more of the Ivy from the comic books and what we really enjoyed," Wynbrandt explained. "What's been really cool about her character, and Maggie [Geha] was so great, was you had Clare [Foley] for the first iteration be the child who is trying to find her way in the world. Then when she evolves into the second iteration of Ivy played by Maggie. She is amazing and embodied sort of a fun kind of comic and zany quality to her who then starts to see that she wasn't being respected so she wanted to do something about that.
"In this third evolution you're going to see a really self possessed,
really scary and driven version of the Ivy character, who is intent on
kind of making the world in the image she sees it should be, which is
the green world. The world where the plants rule and people are an
So based on this, they wanted to portray the transformation of a cheerful Ivy into a unhinged version and thus went with the recast...