I am not sure how you looked up the Ext.NET version off the DLL file but, I just grabbed a fresh NuGet package (off nuget.org) for Ext.NET v4.5.1, and queried it via Visual Studio properties and also opened the NuGet package's folder then opened Windows Explorer's property page for the Ext.Net.dll file, and all I can read around is 4.5.1
Couldn't it be that you downloaded and installed the NuGet package, but referenced your project to an own Ext.NET build you might have done back to version 4.5.0? Take a look on the path it says for the DLL, if you're using the NuGet version, it should be pointing to your project solution's packages
I took screenshots of the property pages, take a look:
Notice the full path informed here (in my "Playground" solution) is:
command (query the variable's value), which should read
(a string). At the same time, still from client-side, you should get version
if you query
Hope this helps!