The redsn0w 0.9.6 b4 could be used to jailbreak iPhone 3GS 4.2.1 iOS. If you want to jailbreak iPhone 3GS on iOS 4.2.1, then here are the steps to do it using redsn0w 0.9.6 beta 4. But remember, for the New Bootrom iPhone 3GS, this is a tethered jailbreak. If you want to SSH into your device, you need to put your device on a jailbroken state on every reboot.

Jailbreak iPhone 3GS on 4.2.1 iOS

1. Download iOS 4.2.1 for iPhone 3GS from here.

2. Update your iPhone 3GS to iOS 4.2.1 using iTunes.

3. Download Redsn0w 0.9.6 b4 from here.

Now follow the rest of the steps given here to jailbreak iPhone 3GS on iOS 4.2.1.