Going back to 3.8
 

Is it possible to go back to 3.8 after upgrading to 4.0?

Did you make a backup of your db prior to the upgrade? If you did, then you can roll back to 3.8. Otherwise I am afraid that you can''t.

I'm pretty sure i did but it was months ago...

A backup is the only way or....
if you can import your 4.0 site to another script, phpbb for example, then you can import the phpbb forum to a 3.8.

This is the only "workaround" I've seen without using a backup that you made prior to upgrading to 4.0 as suggested.

Well, as far as i know, the only whay is, to convert to another produkt and converting back to 3.8 then. a downgrade ist not possibe.....as far as i know

A backup to your database is the only solution "supported" by vBulletin. Importing to another product and back may cause havoc to things such as attachments.

Why do you want to go back to 3.8?