Copy a 4mm tape created on an AS400 to my hard drive
 

What is the command I should use to download a labled file created on an AS400 to my hard drive?

That depends on how it's written to tape. In most cases it will probably be a tar archive. Or multiple tar archives on tape.

You should use the mt command to position the tape to the right location and the tar command to extract files from the tape. You can read the man pages to find out more on how that works.

In general you would do something like this to extract the second archive on tape:

# rewind tape
mt -f /dev/nst0 rewind
# skip first archive on tape
mt -f /dev/nst0 fsf
# extract archive on tape
tar xvf /dev/nst0