Is there such a thing as the fourth state of matter?

I was watching Discovery Channel today and heard something I thought I didn’t hear correctly. The narrator mentioned that plasma is regarded by scientists as the fourth state of matter.
I know matter is anything that has mass or weight and occupies space. I also know that matter exists in three states – solid state, liquid state and gaseous state.
Can matter really exist outside the three forms of solid, liquid and gas? What exactly is plasma as a form of matter?