For iPhone 4 users on T-Mobile (back when AT&T had the US monopoly) or international carriers, 4.2.1 represented a sweet spot. The baseband 01.59.00 was unlockable via the ultrasn0w tool. Redsn0w 0.9.6b6 allowed you to jailbreak without touching the baseband, preserving your ability to use any SIM card.
The "b6" in the version number denoted that this was the sixth beta revision of the 0.9.6 series. It wasn't the final version, but it was arguably the most stable release for users sitting on iOS 4.2.1 during that specific window of time. redsnow 0.9.6b6
This tool is obsolete for modern iOS versions and devices. It is only useful for vintage hardware preservation or hobbyist tinkering. For iPhone 4 users on T-Mobile (back when
Disclaimer: Jailbreaking is done at your own risk. These tools are provided for educational and legacy restoration purposes only. If you're tackling a legacy device, tell me: What is it (e.g., iPhone 3GS, iPod touch 2G)? The "b6" in the version number denoted that
Do you have an old iPhone 4 running this exact jailbreak? Share your memory of the 4.2.1 era in the comments below.