Difference between .net 4.0 and .net 4.6

Hi.

I have a question about using the Rebex package.
I added the Rebex component as a reference in Visual Studio 2017.
I'm using the project's .Net version set to 4.8.

When I add a reference in Visual Studio, the default path of Rebex's dll is set to net-4.0.
I need to specify the HintPath to make net-4.6 in the csproj file.
I wonder if this is the correct way.
Also I would like to know the difference between 4.0 and 4.6.

Thank you.