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:  

• Upgrade DNN Slidedeck 
• Upgrade DNN Video

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.  

Hi

You don't give too much information about the Version 4 site

Questions to ask yourself
1. Home many pages are there
2. What modules are being used
3. What knowledge do you have of SQL

You can then size the job
1. If it's only a few pages just consider doing a new install of DNN
2. If it's only a few modules then look to the latest version and replicate on a new DNN
3. If you know SQL then there is a chance or copying "content" with a few commands

1. If it's basically just pages of "static" content manually copying up to 100 pages may be quicker then going through an upgrade - https://docs.dnncommunity...ggested-upgrade-path
2. If you've only got a few forms (10) consider rebuilding

I've developed something I call DNN Simple which only uses 2 modules Reports and DataSprings Dynamic form. Not even HTML, and I've migrated former HTML paged sites to this quite easily.

Best of luck

Dave