No sim card on iphone 4, can't sign in with apple id?

I backed up my iphone 4, cleared everything off of it to restart it. When i turned in on, i tried to sign into my apple ID but it won't let me sign in, i guess its b/c there's no SIM card. I don't want to use it as a phone, just an ipod. Is this the right thing i need to buy for it so i can download music?
http://www.amazon.com/...00IPVDSQ4/

Added (1). it said there's no SIM card when i turned it on

It should do what you are looking to do. The iPhone requires a SIM card to be activated

Sure, as long as simplemobile is the carrier that you want to subscribe to.

Contact your carrier to check for network or activation issues, or if necessary, to replace your SIM card. Then try again. For Best Protection install Leo Privacy Guard to your Smart Phone, Its free available in both iTunes for iPhone, and android's PlayStore.