Is it possible to downgrade modemfirmware? I've updated the iOS on my iPhone 4 to 5.0.1 the baseband was also updated to 04.11.08. I've downgraded the OS it but the baseband didn't change, can someone help please?
In general, it's not possible to downgrade the modem firmware / baseband software version on iOS, even if you jailbreak the device or use software designed to install modified software on an iPhone. 

It's a popular request, so I imagine many people are working on it but nothing is out today to perform the task you ask on the iPhone 4.