Sony removed the download of the new FS5 4.0 firmware. Cinema5D got an email from Sony explaining why they had to remove the firmware update:
Sony has temporarily suspended the new firmware updates for PXW-FS5 and PXW-Z150 due to a minor issue found during testing within the HDR function in the Picture Profile menu. These will be fixed in a new firmware version, v4.01 for the PXW-FS5 and v2.01 for the PXW-Z150, which will be released in the coming weeks.
If the most recent firmware version has already been downloaded we advise to download v4.01 or v2.01 as soon as it becomes available. If there are any further questions please contact the Prime Support team on 00800 7898 7898 and they will be happy to answer your questions.