I am assuming that you have copied the appropriate values/sections (connection strings [BOTH], machine keys, codeSubDirectories, objectQualifier and dataBaseOwner) from your existing web.config file into the version 4.8.4 web.config file.
Although if you are upgrading from DNN 4.5.5, I would have thought that they would have been previously removed, check if the following two dll files are in the /bin folder:
bin\DotNetNuke.Modules.Html.dll
bin\DotNetNuke.Modules.Html.SqlDataProvider.dll
If either or both legacy files are present, delete them and then try accessing the portal again.