Brake fluid: Whats the deal?

[Ray Kunst <RKunst@C-K.com>]

Hi all,
I just had my brake fluid replaced on my 1979 MGB.
I told the Mechanic 
that I was told to use Dot 4 Fluid from the people in my
MG Car Club.
He showed me that the Master Cylinder had the phrase USE ONLY DOT 
3 BRAKE
FLUID. 
I called up tech support (the next day) at Moss Motors. 
Tech Support told me to use only DOT 3. 
Who is right? 
I now have DOT 3 in my car. 
Should I reflush and replace with DOT 4 or should I leave well enough alone?
Inquiring minds want to know.