Upgrading Idea
 

Am total noob here and I haven't upgraded my vb since I started using 3.8.3 few months ago.

I pre-ordered the new 4.0 Suite, and still holding an active license for 3.8

Now my question is

When 4.0 is out,
Can I install it in "beta" folder in my root, letting it pull the same MySQL and play with it slowly and gradually until am satisfied then to move it to the root after deleting 3.8

Is that possible, having two vb installations working on the same database and residing in the same domain?

You cannot run it using the same database as your 3.8 site. You may make a copy of the 3.8 database and then use that for your test site and upgrade that database to 4.0 and play with it though.

So then I should install 4.0 as an upgrade even if it is in a different folder using another (copy) database.
So I cannot just upload it as a clean installation.

Am I right?

You may upload vb4 as a clean installation and then play around with it.

Yeh, but I want to play around with it using my existing database

Then do as I said and make a backup of your live one and import it into a new database and point your test site to it. You cannot run a vb4 installation and a vb3.8 installation off the same database at the same time.