It sounds like you're using grub from a different OS. If that is the case, then vmlinuz is on a different partition then the "/boot" directory of that OS. You could explicitly tell grub where the partition is located like this:
Originally Posted by digitalfriction
This has worked for me in the past with grub and previous installations of BT. This should work if the "/boot" directory from BT4 is located on the same partition as the "/" directory for BT4. Otherwise, change accordingly.
title BackTrack 4 Beta
kernel (hd0,4)/boot/vmlinuz rw root=/dev/sda5