Hello,
I would suggest you test upgrading in a localhost testing environment first.
If possible I would then test that DNN4 is compatible with your hosting provider & make sure you can actually operate DNN4 with your current hosting setup.
For the actual upgrade, I haven't tried this yet, but I would suggest you check out the free
upgrade video as the process will be very similar. Once you have learnt the upgrade process, I would then study how to install and configure your hosting provider for DNN4. The 2 pieces of information should then help you to set this up.
The most important thing is to test this out before trying it on a live website. The other thing to bear in mind is that I have heard that DNN4 is not as stable as DNN3 yet, so do you really need to upgrade?
Hope this gives you some starting points, thanks,