EPC light on

Not good - Read the owners manual you are supposed to take it to a dealer. I had a whole 6 months of re-occuring EPC light probs when I first got my S4. Has the car lost power?

The EPC light is telling you that there is a an engine management prob. Typically some sensors of which there are many.

In most cases the car will go into what is often referred to as 'limp mode'. Basically the ECU shuts the fueling/boost down to a moderate level.

You don't say what type of A4 you have in your profile.

if you don't have VAGCOM or know someone with VAGCOM or similar you will have to take it to either a VW/Audi specialist or dealer to have it diagnosed.

Let us know how you get on. Good luck