As far as solid, liquid, gas, plasma go, why is plasma the highest state?
Are there any other states of matter?
If by highest, you mean temperature (proportional to mean kinetic energy of the particles), then the plasma state is "higher" than the other states you list.
I think that there are other "higher" states of matter. For example, when it becomes energetically favorable for protons and electrons to combine into neutrons, you get a state called "neutron degenerate matter". (By the way, have you ever read "Dragon's Egg"?)
An even "higher" state would be QCD matter, e.g., quark-gluon plasma.
The highest state in terms of what?
There are lots and lots of states of matter. Personally, I like Bose-Einstein Condensates, but just because it's so fun to say.
Not all materials fit into the 3 classic states. Plasma is only one of many. Glass, for example, isn't really a solid, but it's not a liquid either.
Wikipedia has a nice description of the states of matter