I just recently bought an iphone4. It came installed with 4.2.1 firmware, and since 4.3.4 is released now it is not as simple as to just update the firmware with itunes. I have tried some good which tells you to use tinyumbrella, but I didn't got it to work. As I got error message 1604 when trying to restore. Anyone know how to upgrade to 4.3.3 firmware?
TinyUmbrella is probably stopping you from updating your iPhone, because that creates a service that redirects port requests to itself rather than the Apple servers. I do not recommend using TinyUmbrella for anything unless you know exactly what it does and how to use it.
The reason you are having troubles is because you are probably trying to install the wrong IPSW file onto the iPhone.
Download the latest iTunes and install it (this will automatically update it) and then connect your iPhone, and in iTunes, request it to check for any iOS updates.
iTunes will download the required IPSW, get a certificate request from Apple servers and then install it. Problem solved.