From the TechNote you referenced
Installation Procedures:
1. Install Service Pack 4, found in the Related Documents section below, and reboot
2. Download and extract IDR_Prep_278553.zip
3. Copy the files manually into the correct locations:
The readme.txt file should be excluded.
The bedrwiz.ini file should be placed in the \Program Files\Veritas\Backup Exec\NT\idr\disk4 directory.
All other files should be placed in the \Program Files\Veritas\Backup Exec\NT\idr\disk1 directory.
· 4. Install VERITAS Backup Exec (tm) 9.1 - Device Driver Installer (20050115), found in the Related Documents section below, and reboot
After reboot, the IDR Prep wizard should run and complete correctly.
SP4a is merely SP4 repackaged to supposedly bypass the installed service check (which still fails) so these instructions would apply to SP4a as well as SP4