The latest update adds in level 4, Phoebe Peleus, which I've taken a look at in the below video:
They do have more than 4 levels though, since they also have the challenge mode which is incredibly satisfying to play on the higher difficulty levels.
On top of the new mission, they've also added in some ambient sounds to the missions, to make them that little bit more engaging. Your ship has a new exterior, you can now select which "controller" is primary, many gamepad/joystick improvements, the Holo-Guide will no longer show you secret power-ups you haven't yet found yourself and many more improvements.
Overload has also seen some optimizations and plenty of bugs fixed with this latest update and it's smoother than ever.
See the full changelog here. When finished, it's going to be an incredible game considering how fun it is already.