There are definitely more than four distinct states of matter. Even among solids, there are hundreds of different crystal structures, and conversion between them can be just as dramatic as the conversion between other states. For example, pure tin tends to crumble as the atoms rearrange from one crystal form to another.
Even if we lump all the crystalline solids into one class, there are still plenty more besides gas, liquid, and (I'm guessing this was your fourth) plasma. For example, there are a variety of different liquid crystal phases, used almost everywhere in electronic displays. In these phases, some aspects of the molecular arrangement (say the organization into sheets) are regular, like in a crystal, but others (say the molecule positions with each sheet) are disordered, like in a liquid.
In some important ways, various types of magnets, superconductors, etc. are also distinct states of matter.
(published on 09/18/2008)