LTO3 to LTO4 Tape Migration

Another 2c from my side....

If your management really requires you to duplicate 400 tapes, they should provide you with additional resources:
1. Manpower - a dedicated resource who will only perform duplications for the next couple of weeks.
2. Media server - Best to dedicate a media server that will ONLY be tasked with duplicates
3. Tape drives - as many source (LTO3) and target (LTO4) tape drives that can be dedicated to this task and presented to the dedicated media server. (I would request 2 LTO3 and 2 LTO4 tape drives connected/zoned to 2 different hba's on the media server to ensure performance.)

'Duplication person' can then break the task in to smaller 'chunks'. Choose 5 - 10 tapes and run 'Images on media' report (bpimmedia).
Add all image-ids to a text file. 
Run bpduplicate to '-dstunit <destination storage unit label>' with -Bidfile parameter (text file created above).
If you have more than one media server, you will need to specify the dedicated media server as 'alternate read host'  (-altreadhost <hostname>) to prevent duplications across the network.
Add '-primary' to change new copy to primary.
Retain multiplexing as de-multiplexing will slow down the process considerably.

Duplication person to verify successful duplication of all images on LTO3 tape before expiring source media.