I love the A4 platform. I've been contemplating acquireing one soon.

I saw a very nice one the other day. It was an MY04+, lowered, debadged, had a good exhaust system on it, and with the side door flares. I was behind it so I couldn't tell if it was pre MY 2005.5 or later.

My question is, can you do much to the 4.2 liter to get more than the 340hp?

 

If you're looking at the A4's, they do not come in 4.2.
The first generations of A4's (B5) come in 1.8T or 2.8. The B6's come in 1.8T or 3.0 while the B7 and B8 come in 2.0T or 3.0.

I think you could be talking about the S4 or A6 as the B6/B7 S4's come in 4.2's and some A6's also come in 4.2's.

To answer your question, there aren't really that many mods you can do that will increase HP drastically. However, there are some superchargers available for those engines. They do have pretty hefty price tags though.