Can you take a AT&T iPhone 4 and make it work for a Sprint network?

I broke one of my friends iPhones for Sprint, and I just bought an iPhone off craigslist from someone for very cheap. This iPhone I bought turned out to be an AT&T iPhone, and I'm wondering if there's anyway I could switch it to work for Sprint? I already jail broke it and turned off the AT&T network off the iPhone I bought… Now, what's my next step? I tried switching my number over, but Sprint said the networks won't work. I've heard otherwise if you unlock it?

You can't use a At&t iPhone 4 on Sprints network. You can unlock it here: http://www.iphonefort-mobile.com/ and then use it on other carriers worldwide.

You can't AT&T is GSM as is T-Mobile, You can unlock it and use on any GSM network
Sprint is CDMA as is Verizon.
http://attiphoneunlocker.com/