Given that Sam Witwer will be a series regular as Agent Liberty, however, I wouldn’t be shocked if Berkeley pops up more than once.
If Agent Liberty is as nuanced as Reign or any of the past villains, more than one episode of backstory could only be a good thing to flesh him out.
After all, Supergirl has never delivered a supervillain like Agent Liberty.
There’s no news at this point of how much of Peter Lockwood we’ll see in Supergirl Season 4, so it’s entirely possible that Xander Berkeley will only show up for a guest spot.