I\'m currently developing an application for .NET 4 Client Profile, as this is the version that will be present on most home computers through Windows Update.
Howeve
Even if you deploy the copy of the System.Web.dll to the client that won't solve the problem. And the problem is: you can't link to the assemblies compiled for .NET 4 profile (not client) from the assembly that compiled for .NET 4 Client profile. The only solution is to target your assembly to the not client profile.
The MSDN Page about .NET Framework Client Profile.