So i have an APR stealth exhaust on my 2004 audi a4. I want to put a downpipe on it. Do I need a certain downpipe to fit the exhaust or will any one made for my car work?

 

As you said that you want to change your downpipe of exhaust system, let me know you, if you change your down pipe then the diameter of your newer one down pipe must be same as ur older one down pipe because if you changes the diameter, the performance of exhaust gas would be changed which will affect the performance of the catalytic converter.
Second thing, if the bend in the new down pipe is narrow or wider than original pipe, the back pressure on engine will increase; which will reduce the power output and may damage the exhaust system parts.
You should keep the above instruction in mind at the time of changing the exhaust down pipe.