Not sure if this is posted but here goes.
Basically all you have to do is the public game exploit that was posted on these forums, and make a game on Act 4 inferno Prime evil, Enter the Crystal Arch.
1. run up to the shrine and click on it.
2. Talk the the ghost and move back.
3. Let Tyrael tank the boss and you can dps it abit if you'd like. Tyrael does around 50k dps.
4. Get a blue or a rare on Act 4.
It is sort of luck based in that you ideally want the maghda ghost, but it is still doable. I would recommened using mf gear, so your chances of getting a rare are increased.
Video:
Diablo 3 Inferno Act 4 farming - YouTube