nuget package manager does not offer older versions of package

I inherited some old legacy code which I'm trying to build, but I'm missing some specific dlls. In particular, I need version 4.0.0.0 of System.ComponentModel.Composition, but the Nuget Package Manager options only go down to 4.5.0. Why is this, and how can I get the version I need?

Here is the line specified by the Web.config file I inherited.

<add assembly="System.Net.Http.WebRequest, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A" />