To start using the FEL v4, you'll first want to
download the NuGet Package Manager. It's a Visual Studio extension that will help manage the libraries used by your project. NuGet will, by default, include a reference to the NuGet official package source, but you'll also need to add a reference to the FDOT repository as well.
Managing NuGet Packages Using the Dialog gives an overview of the NuGet dialog and, under the topic
Package Sources, also gives specific instructions on adding a new package source. The source you'll need to add is
https://nuget.fdot.gov/nuget.
Once you've added the package source, you can open the Package Manager and select the FDOT NuGet Repository to view the available packages. Selecting one and clicking Install will add the package and any dependencies to your project.