So I want to find out what's in /dev/sda4 on my hard drive. The
computer has *never* had Windows on it. So I try to mount it, and am
told:

april:/farhome/hendrik# mount /dev/sda4 /test
NTFS signature is missing.
Failed to mount '/dev/sda4': Invalid argument
The device '/dev/sda4' doesn't seem to have a valid NTFS.
Maybe the wrong device is used? Or the whole disk instead of a
partition (e.g. /dev/sda, not /dev/sda1)? Or the other way
around?
april:/farhome/hendrik#

Why would it try for and NTFS file system on a Linux machine?

-- hendrik