Suite Answered Upgrades & Fresh Installs
 

ALOT Of you have problems* i'll start with the installs.
All files MUST be uploaded* if you get an error with the file name* reupload it.
For upgrades from 3.8.x & lower have ALL modifications removed even their files including XML's php files images and what not.
If you forgot something* make sure you disable all hooks via config.php. Also ALL vB 4 files are upgraded* with that you will have a successful install OR upgrade.

USEFUL Topics:
How To Backup Before Upgrading

vBulletin 4 Minimum System Requirements

Hopefully that has helped you.....