Brake Fluid: Dot 3 vs Dot 4?

[Dwayne Cosby <dccosby@interlog.com>]

Quick question.

I've been meaning to change my brake fluid for a while (ever since my last
track day the pedal has been very spongy so it needs bled at the least). 
The advice of others at the track tells me that Ford Heavy Duty fluid is
the cheapest and one of the best fluids available (apparently it is also
resold by Performance Friction). So I bought a liter for $7. 

I see from the packaging that it is Dot 3. My S4 manual specifies Dot 4.
What is the difference and what are the risks of using it?

Thanks for the advice and Merry Christmas.
Dwayne Cosby
94 S4