Forza 4: Speed Demon Achievement Guide

This achievement is unlocked when you hit 240mph in any game mode. Here's how to do it.

Forza 4: How to unlock the Halo Warthog

In Forza Motorsport 4 there is a mode called autovista. You can look around and inside cars whilst a commentators gives you interesting facts about them.
This is where you unlock the Warthog.

Forza 4: Easy Credits Exploit

You will need access to the benchmark test tracks for this and also Xbox live gold to use this incredible exploit.