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