By definition, you don't "upgrade" from .NET 3.5 to 4. You just add the .NET Framework 4.0 alongside with the other versions. So, it is farely risk free, yes.
QlikView Server 11 have some components using 3.5, but does not require 4.0 unless for the components running under IIS, if that is used as web server for the QVS.
A tip; make sure you have 4.0 installed prior to installing QlikView Server 11, especially before configuring IIS. That will save you some binding troubles in regards to ASP.NET and IIS.