[Carnage]

Heres what i suggest you do (and is what i'm doing for my board on a MUCH MUCH bigger scale (1.2million posts 100k threads 30k members)

For your live board
1. remove any products, styles and other superfluous stuff from your installation. For me this involved removing some test skins; uninstalling a few disabled products; cleaning some old db tables out of the database and removing old files from the install directory. (the check for suspect file versions helped with the last one)
2. Make announcements to your staff and members about the upgrade to vb4

For your test (1) board:
1. Create a test board, on this install a CLEAN copy of vbulletin 4.
2. Find versions of all the plugins/products you wish to keep and install them on this board (ensure they work)
3. Convert your templates and styles on this new board .
4. You basically need to make this board identical in functionality to your main board.
5. Invite staff and some select members over to this test install to ensure its working correctly.

For test board (2):
1. Make a copy of your database and file system from your live board. (it may be necessary to take the board offline while you do this)
2. DOUBLE CHECK that this is a copy and its pointing to a different database.
3. Perform a test upgrade to vb4 on this test copy.
4. Import styles and products that you tested on test board (1).
5. Invite staff + select members to try and break this test board as well.

Repeat the above as many times as needed until your test (2) board is upgraded successfully.

On your live board:
1. Shut down board
2. Make a new database and file system backup.
3. Upgrade to vb4
4. import products and styles from the working test board (2)
5. Reopen your board.

After its back up, remove your two test boards.