My sort of method for Cricket/MetroPCS Flash here:

Initial steps to flashing your iPhone 4 CDMA (Verizon) to Cricket or MetroPCS (speculative?)

1) Jailbreak your iPhone 4 on the 4.2.6 firmware using the latest version of GreenPois0n at greenpois0n . com
2) Download i-Funbox www dot i-funbox.com
3) Connect your iPhone 4 to the Windows PC and the modem drivers will load up
4) Open up i-Funbox and then check to see whether your iPhone is connected.
5) Scroll down to Raw File System then open up the directory called: [iPhone]://System/Library/Carrier Bundles

(speculation from here since I don't have my own iPhone 4 yet)

6) Erase the Verizon prl.
7) Cut and paste or load up the Cricket/MetroPCS prl you want to use by using Actions then Copy from PC tab on i-Funbox.
8) Close up and restart the iPhone
9) Activate using *228 or *22804 to get working talk and text?