Downgrade iPhone 4 baseband from 4.12.01
There is no way to downgrade the baseband on an iPhone (or any Apple device, the iPad being an acception but also rife with disaster).

The only way to downgrade the baseband is to install a previous version that is still being signed by Apple. For example, if they release 5.2 today, they would be signing 5.1 and 5.2. If 5.2 came with a newer baseband, it would be upgraded once you've installed that version. If you decided to revert immediately to 5.1, it would downgrade both, the OS and the baseband. 

Apple typically leaves the signing window open for only a few days. Once it's closed, you cannot alter the baseband (even if you successfully downgrade the OS).