The original activation servers for 2004-era products are generally no longer online. This means even with a valid serial number, the automated "Authorization Code" generator within the software may fail to connect. Official Policy:
If you need AutoCAD today, Autodesk offers a of the latest version, and monthly subscriptions start around $245/month (or $55/month for AutoCAD LT). Autocad 2004 Serial Number And Authorization Code
Some universities may retain old lab licenses, but these are not available to the public. The original activation servers for 2004-era products are