JailBreak iphone 4/3GS to iOS 4.2.1

Update on 4/4, my iPhone 4 iOS 4.21 was jb by the same greenpoison file successfully.

Just safely upgraded & JB my 3gs to 4.2.1, which looks no diff from 4.1..

1. download iOS 4.2.1 fw + greenpois0n
2. iTune: shift + "Restore" --> select fw path --> upgrade ios
3. turn off iPhone, quit iTune, and then jb by greenpoison.exe (new ip4 takes very long time, as if device hang, just dont touch it
4. install Cydia: iphone click "Loader"
5. remove loader from iphone: loader --> option
6. disable screen auto lock, let Cydia do 1st time check, do not disturb until reboot done.
7. Cydia add source: http://cydia.hackulo.us
8. Cydia add source: http://www.sinfuliphonerepo.com
9. Search and install AppSync for ios 4.2.1
10. if afc2add was not installed by greenpoison automatically, install from modmyi, this enables installation of sj.91