Can someone explain or walk me through the reasoning for this error:

Could not load file or assembly 'VistaDB.Web.4, Version=4.1.0.0, Culture=neutral, PublicKeyToken=dfc935afe2125461' or one of its dependencies. The system cannot find the file specified.

I shouldn't need to install vista db and if I did, where would i find it??  why isn't it with the install of DNN??