Thank you for posting your question. It is possible to run different versions of Polycom firmware based on the model. When upgrading or downgrading between Polycom 3.x to 4.x you need to use the ADTRAN/Polycom IP Phone Firmware 4.0.x Upgrade and Downgrade Guide to make sure the bootcode is updated and the proper configuration files are generated for that version of firmware as the syntax changed between those versions. Here is a link to the IP 550 firmware page on our main website for the files you will need to accompany the guide.
If you still have problems after going through those steps, I would need to see a copy of your configuration along with the output of a debug ip ftp-server when one of these phones is rebooted. You can submit that to our FTP server with the instructions below. It would also help to know all the messages on the display of the phone while it is attempting to boot.
FTP upload instructions
Open Internet Explorer web browser on their PC
Type the following URL: ftp://ftp.adtran.com
Press the Alt key, click View, and then click Open FTP Site in Windows Explorer
Double-click the "Incoming" folder
Drag and drop files from PC into the Internet Explorer window
Reply to this post with the exact filenames used so we can retrieve the files