I would also verify that nst0 is assigned to your LTO 4 drive What brand of Ultrium LTO 4 do you use. Mine is a Tandberg Data or Exabyte. I use the ltoTool to check and see if the device is being recognized. If your Ultrium LTO 4 is not an Tandberg, I am sure the manufacturer has a tool to verify the device. My ltoTool command and output is:
fs2:/ # /downloads/tandbergtools/Linux/ltoTool -i /dev/nst0 Tandberg ltoTool -- V4.90t Copyright (c) 2006-2007, Tandberg Data ASA. Copyright (c) 1996-2006, Exabyte Corp. *** BETA *** BETA *** BETA *** Test version! Use at own risk. This beta version built by RTist BV - The Netherlands on Oct 11 2007, 23:22:05 Tape Drive identified as LTO4(IBM) /dev/nst0 - Vendor : IBM /dev/nst0 - Product ID: ULTRIUM-TD4 /dev/nst0 - Firmware : 74H6 /dev/nst0 - Serialnum : 1310082873 /dev/nst0 - This tape : 5 times loaded into a drive Done