Forum Answered My Experience with upgrade from 3.8.5 to 4.0.3
 

I was waiting for upgrade from 4.0.0 but after reading experience on the vbulletin forum, I was bit confused. So I have created a test forum and backup of my database. It went very smoothly. No problem at all. So If somebody is waiting for upgrade, just take back up and disable plugin and upgrade. And enjoy 4.0.3.