Can I use unlock an at&t iPhone 4 to work with sprint and then flash it?

I have an iPhone 4 that I want to flash to clear talk but they can't flash it on at&t so I was wondering if I could possibly unlock the phone, put it on sprint, then take it to clear talk and then have them flash it.

Okay you officially confused me with the flashing/unlocking talk. But to answer your question in the title no you can't.

An unlocked AT&T iPhone will only work on gsm networks. Sprint is a cdma network. I have no idea what clear talk is but if it's a gsm carrier then their SIM card should work in an unlocked phone.

A few other things:
Flashing is a term used for cdma networks
Unlocking is a term use for gsm networks.

You can't.

AT&T = GSM
SPRINT = CDMA

FLASHING = CDMA
UNLOCKING = GSM

FLASHING + AT&T = NO
UNLOCKING + SPRINT = NO

UNLOCKING + AT&T = YES
FLASHING + SPRINT = YES

To clear the talk you can go to nearest store and flash it… But to Unlock I-phone from AT&T USA Network lock use remote Unlock service… Visit the site http://www.theunlockarena.com/apple-iphone-4/rs13wp9/ get the remote unlock service at reasonable cost and Unlock it easily… But You can't use with CDMA Network after Unlock it… You can use only with gsm network sim card… Sprint is CDMA Network You can't use with Unlocked I-phone 4. After Unlock it use with any GSM Network like Cingular, T-Mobile etc…