can i trade in my iMac G4 for a MacBook pro or an iMac

No, apple does not do trade ins. There is a private company that offers a recycle program for old Macs (they give your store credit for it) but they are a for profit private recycler that Apple contracts with.

http://www.apple.com/recycling/

The Apple Support Communities are an international user to user technical support forum. As a man from Mexico, Spanish is my native tongue. I do not speak English very well, however, I do write in English with the aid of the Mac OS X spelling and grammar checks. I also live in a culture perhaps very very different from your own. When offering advice in the ASC, my comments are not meant to be anything more than helpful and certainly not to be taken as insults.

No trade-ins with Apple. That Mac has little street value. Sell it and use what little that you get towards a new Mac.