You can download the iPhone recorder for your iPhone 4 and install it to your phone. The app is very easy to use since it just pops up whenever you make an outbound call or whenever you receive an inbound call on your iPhone 4. The recorded conversation can then be saved and sent as an email attachment.
None of the solutions above work to record both incoming and outgoing calls. There is only one solution that doesn't have any strings attached, and it's an actual recorder that you plug into your iPhone: