How can DNN upgrade from version 4 to the current version 9?
Well, the short answer is this...
Get to know the general DNN upgrade documentation.
Use the suggested DNN upgrade path as your starting point.
You'll find yourself going through a loop for various versions by first upgrading every extension you can, then upgrading to the next version of DNN you find you can safely upgrade to. You may find yourself having to contact various module vendors to get several versions of their extensions, so you can upgrade to the current release. It will be a lot of upgrade DNN, stop to upgrade extensions, upgrade DNN, upgrade extensions again, etc.
This is further explained in my most recent DNN Upgrade presentation:
Now, realistically, depending on how complicated the site it, you may find your time better used by using an alternative method/suggestion.
This is the time when I would be looking at the website in more detail, and then suggest that we instead build a new DNN instance that is using the latest modules, then import all of the content into these newer modules. These newer modules may even be different modules altogether from the previous website. It's possible you're using extensions that no longer exist since DNN 4.xx. It usually results in a more stable website, and takes a shorter amount of time.
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