what are the four types of channels and how do they differ?

Answer 1

Could you be a bit more specific?

If you are refering to types of gated ion channels you have
1. voltage-gated
2. ligand-gated (extracellular ligand)
3. ligand-gated (intracellular ligand)
4. mechanically gated

If you are refering to the size of the pore you have mainly 3:
1. Voltage gated (4 subunits)
2. Transmitter-gated (5 subunits)
3. Gap Junctions (6 subunits)

the size of the pore increases with the nuber of subunits. In plants you have plasmodesmata instead of gap junctions and in bacteria you have porins (3 subunits, the smallest pore from the aforementioned)

I am not aware of any other type of channels.

another classification is according to TC-DB
(http://www.tcdb.org/tcdb/ )

"Channel-type facilitators. Proteins in this category have transmembrane channels which consist largely of a-helical or b-strand-type spanners. Transport systems of this type catalyze facilitated diffusion (by an energy-independent process) by passage through a transmembrane aqueous pore or channel without evidence for a carrier-mediated mechanism. They do not exhibit stereospecificity but may be specific for a particular molecular species or class of molecules.
These include:

1.A. a-Type channels. Transmembrane channel proteins of this class are ubiquitously found in the membranes of all types of organisms from bacteria to higher eukaryotes. These transporters usually catalyze movement of solutes by an energy-independent process by passage through a transmembrane aqueous pore or channel without evidence for a carrier-mediated mechanism. These channel proteins usually consist largely of a-helical spanners, although b-strands may be present and may even contribute to the channel. Outer membrane porin-type channel proteins are excluded from this class and are instead included in class 1.B.

1.B. b-Barrel porins. These proteins form transmembrane pores that usually allow the energy independent passage of solutes across a membrane. The transmembrane portions of these proteins consist exclusively of b-strands which form a b-barrel. These porin-type proteins are found in the outer membranes of Gram-negative bacteria, mitochondria, plastids and possibly acid-fast Gram-positive bacteria.

1.C. Pore-forming toxins. These proteins/peptides are synthesized by one cell and secreted for insertion into the membrane of another cell where they form transmembrane pores. They may exert their toxic effects by allowing the free flow of electrolytes and other small molecules across the membrane, or they may allow entry into the target cell cytoplasm of a toxin protein that ultimately kills the cell. Both protein (large) and ribosomally synthesized peptide (small) toxins are included in this category.

1.D. Non-ribosomally synthesized channels. These molecules, often chains of L- and D-amino acids as well as other small molecular building blocks such as hydroxy acids (i.e., lactate), form oligomeric transmembrane ion channels. Voltage may induce channel formation by promoting assembly of the oligomeric transmembrane pore-forming structure. These depsipeptides are often made by bacteria and fungi as agents of biological warfare. Other substances, completely lacking amino acids, are also capable of channel-formation.

1.E. Holins. Holins consist of about forty distinct families of proteins that exhibit common structural and functional characteristics but which do not exhibit statistically significant sequence similarity between members of distinct families. They are encoded within the genomes of Gram-positive and Gram-negative bacteria as well as those of the bacteriophage of these organisms. Their primary function appears to be transport of murein hydrolases across the cytoplasmic membrane to the cell wall where these enzymes hydrolyze the cell wall polymer as a prelude to cell lysis. When chromosomally encoded, these enzymes are therefore autolysins. Holins may also facilitate leakage of electrolytes and nutrients from the cell cytoplasm, thereby promoting cell death. Some may catalyze export of nucleases. "

Answer 2

Auditory channel
Visual channel
Somatosensory channel
Gustatory channel