Langdti,
you were right. It was a T40 screw. You know what was the problem? I tried to dismount them with a very cheap torx L-shaped set of keys, and the T40 was slightly bigger than it should have been! I bought a single (good) T40 tool, that fits perfectly with the screws and allowed me to take off the 4 screws with no problems.
Thank you again,
bye!
Greg