Brake fluid question

[Stevemr]

I have a 2007 Audi A4 3.0tdi.
In the past I have always used audi/Vw brake fluid.
Is that necessary though or will any DOT 4 brake fluid do?
Also I usually pump about a litre through every 2 years, is that enough to change the fluid?

[cuprabob]

Any Dot 4 fluid will do.