Untethered 4.2.8 Jailbreak Verizon CDMA iPhone4 for Mac Using Pwnage Tool
Unfortunately, neither Redsn0w nor Pwnage Tool are currently compatible with the iPhone4 CDMA firmware 4.2.8 However, @ih8sn0w has released a firmware bundle that will allow you to create a custom firmware on a Mac using Pwange Tool 4.2
Step 1: Ensure you are using Pwnage Tool 4.2 and install it on your desktop. Also extract the firmware bundle from the file provided above. Right-click the Pwnage Tool icon and select "Show Package Contents" and then navigate to Contents>Resources>FirmwareBundles
Step 2: Drag the iPhone3,3_4.2.8_8E401.bundle file into the "FirmwareBundles" folder and proceed to jailbreak with Pwange Tool.
description="Adding 4.2.8 Firmware Bundles to Pwnage Tool"
Firmware Bundles Cannot Be Modified
If you are receiving an error when attempting to import your firmware bundles along the lines of "cannot be modified," please be sure that you have extracted all of the downloads from above to your Desktop(firmware bundle, firmware file and Pwnage Tool application). If you extract the files elsewhere on your computer you may receive a permissions error such as the one listed above. If you extract all the files to your Desktop you should not run into this error. After dragging the 4.2.8 bundle into the corresponding folder from above simply open PwnageTool and select "expert mode" and browse for the 4.2.8 firmware file you downloaded and follow the on-screen steps to create your modified firmware.