So I just bought an iPhone 4, need help/advice.

I just bought an old iPhone 4 off a friend of mine who upgraded to a 5. It's unlocked by AT&T (I have a straight talk SIM in the mail, but it won't get here for at least a week), and is running iOS 6.1b4. So I had some questions - is it possible to downgrade to iOS 6.0.1 and then activate it without a SIM card, so I can use it while I wait for my SIM to arrive, or would it be best to wait until the SIM arrives. It'll be a great upgrade over my CM10 Wildfire S, but it's been sitting on my desk for a week now. Really want to do something with it.