I know that it is possible to install lower versions of IOS to jailbroken devices over various tools by creating custom firmware.The problem is we have some test devices in the company and we want to have one device eactly with IOS 4.3. It will not be downgrade since the device itself has IOs 4.2.1 installed.Is there a documented way to active it and the most importantly where can i find older IOS versions?
Apple doesn't want you to install anything but the latest version available of iOS, so every install of a firmware will get checked with Apple's servers.
There is a way to save the authorization for other firmwares with tools like TinyUmbrella, but you can only save the authorization of the currently allowed version; so, if you did not use TinyUmbrella 2 years ago when iOS 4 was the newest version, there is no way to downgrade your device.