Four didn't feel right leaving without Mia and Conall. She left Rose to leave with WA9 and went out to search.
Sorry Erica...but I think I just killed us both... She solemnly thought as the others left and she set out to find her son and her beloved. She found Charlie's GDM, but that's all she found. She resigned to wait for Mia and Conall near it as long as she could afford to, unfortunately the device was not only poorly powered, but poorly constructed. It malfunctioned and sent her to the new Universe, a few weeks ahead of everyone else.
She spent her time in the new universe mourning her lover and son, until she came across Mia one day by chance. Mia didn't have any memories of Four though, as this Mia was the one belonging to this world, so they had never met before. Four was happy enough to find Mia still alive in some form that she didn't care though. The two fell in love again, and before they knew it Four had given birth to two kittens, who she couldn't help but name Conall and Asha.
Every night before they fell asleep, Four would whisper into Conall's ears, "Always remember Son, that I love you and want to protect you. You should trust me more than anyone else in the world. I want you to always remember that."
She whispered to Asha too, "Don't worry about anything sweety. I'm always going to be here to protect you."