It has been quite some time since iOS 4.3 is available in iTunes and now Apple has decided to block the possibility to save SHSH for iOS 4.2.1 or to restore to that software version without using a SHSH saved in Cydia. After each release of a new version of iOS, Apple blocks the possibility of saving SHSHs for the old one and if you did not have the SHSH saved in Cydia/PC then you can never restore to that version of iOS. iHs8n0w has developed a program that allows us to save SHSHs for the version of iOS installed in our terminal, no matter what it is, but for now he has not released it.
In conclusion, from now on you will be able to restore to iOS 4.2.1 only using a SHSH saved in Cydia/PC. You can follow these tutorials to do this: 1, 2.
