Catra cannot STAND Scorpia in Season 4 because Scorpia still believes in the version of Catra that she had to kill in order to pull that switch.
Catra is haunted by having opened the portal and by having sent Entrapta to Beast Island. She knows she went too far, but she is so focused on winning that she shoves her doubts aside - and with them any real connection to other people beyond utility.
Every time Scorpia offers genuine friendship in the face of Catra's increasing violence, Catra rejects it. Every time she brings up Entrapta or the portal, or Emily wanders in, Catra shuts down anything associated with people she once cared about and then betrayed. She can't afford Scorpia's friendship when it means reconciling how she has broken others.
Catra is the one driving the ship now, more motivated than Hordak is to conquer Etheria - because she has already done the worst thing in pursuit of winning, so she has to make it worth it. Her only options are to see this through or to stop and reflect on her past actions - which means making amends with the people she has hurt. So she doubles down on the hurting.
And when Scorpia finally leaves, Catra breaks - because she can never actually be whole without the part of herself that Scorpia believed in.
