The Xiaomi MIX Fold 3 was released in August of this year, so its successor should be available around the same time next year. That means the Xiaomi MIX Flip, or whatever name Xiaomi chooses, could arrive before then.
Xiaomi could always choose to launch them simultaneously, despite the fact that the Xiaomi MIX Flip advances ahead of the MIX Fold. Another thing to consider is that Xiaomi may choose the MIX Flip 4 name simply to unify the names of the two phones.