OK, I thought you must be talking about Soldier Info GUI... because there is just no more space to put this on the Battlescape GUI.
But maybe someone will come up with a clever trick to sqeeze it somewhere, somehow.
Maybe add a feature to let the modder or player decide which four bars are displayed. But in my plays, I don't use psi attacks enough that I'd feel the Psi Energy needs to show outside of the soldier info screen. I could easily just check the info screen for remaining Psi Energy, same as I do to check their effective psi power before using a psi attack, or to check their remaining armor after they take a hit or before going into line of fire.
I'm sure an artist could find a way to squeeze in a fifth bar, if anyone is up to the challenge. But if nobody does redesign it, I don't foresee it being a major issue.