Hello,
There is not a specific tutorial on this, but this tutorial will help:
How to upgrade DotNetNuke from v3.x to v3.2 on a live websitewhich is free!
The only difference is that for DNN 4.x you need to enable ASP.NET 2.0 rather than use 1.1.
I ran some tests following the same steps as in the tutorial from 3.1 to 4.3.5 and it went through ok on a localhost installation.
I would suggest you try the same first on a localhost installation to test out the process.
I then attempted the same on a live website by downloading a copy of it to my laptop and it didn't go through completely without errors due to some of the modules installed on it.
So if you are upgrading a live site I would suggest that you download it to your system first to run a test upgrade and see how it reacts.
Hope this helps, thanks,