If you don't have five energy in the discard you'll still be able to use it. It's the same concept as Energy Recycler and so on. If you only have four energy than you can only pull four [R] energy out of the discard.
Hope that helps!
I haven't played a whole lot of it yet, but from what I have played it's pretty awesome. I enjoy the roller weapon so far as it's pretty funny to just splat people close up with paint.
Story wise I'd say I'm still a bit of a ways away from completing that too...