Replies:
You are probably asking about the phases of matter such
as liquid, solid gas, right? There are a number of
candidates for "fourth" phase - actually condensed matter
physicists probably know about 20-30 different phases
(determined by magnetic or electrical properties for
example - that is if you count insulators and metals as
different phases). However, I believe the standard answer
is that, as you heat things up, first they go from
solid to liquid, then to gas, and finally to an ionized
plasma. And particle physicists would go beyond that
to describe even higher temperatures when nuclear energies
become comparable to the temperature, but such temperatures
have not been reached here on earth (even fusion reactors
just use the plasma phase and rely on relatively rare
nuclear reactions). Well maybe in some very high energy
particle collisions...
A plasma is just what happens when the temperature is so
high that the atoms start to lose their outer electrons in
collisions, and the plasma consists of positive ions and
negative electrons moving about freely.
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