Brake fluids

[Philippe Habib <phabib@apple.com>]

Can someone provide a quick description of the 3 kinds of brake fluid?
 
All I know is DOT 3 rots rubber and DOT 5 is silicone and while being kind to
rubber and not absorbing water, traps air instead?

What is the DOT 4 stuff?  How important is it that I drain the DOT 3 out of
my TR7 and replace it?  Should I use 4 or 5?  The car is an occasional use
street only car.