Here is a news for Verizon iPhone 4 users who want to jailbreak iOS 4.2.8 untether on Mac OS. ih8sn0w has released a firmware bundle that will allow you to create a custom firmware on a Mac using Pwange Tool 4.2.
Download PwnageTool Bundles iOS 4.2.8 Mac
Step 1: Ensure you are using Pwnag 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.