If I install .NET Fx 4.0 on Windows 7 will it bring me any advantage if I will never need an app that is built for this version?
A properly written installer will install (after asking) the correct version of the .NET framework it needs.
So installing .NET 4 now you'll:
a) save yourself a little time when you do install such a program as you won't have to do that step.
b) guard against an improperly written installer for a program that requires .NET 4 but doesn't go and download it for you.
Each version of the .NET framework is independent, so if you have .NET 3.5 installed any program targeting that version will continue to run. There are separate updates from Microsoft for each version of the framework.
So, installing .NET 4 won't give you any performance improvements or extra security for applications you already have installed.
Every .Net Framework installation also installs all the previous versions from 2.0 and onward.
Version 1.1 is an exception to this rule and has to be installed separately.
Since different products that you might possibly install may have been compiled with different versions of .Net, and will require that their version to be installed in order to function, the best idea is always to install the latest .Net version, currently 4.0. This takes care of all .Net versions from 2.0 and up to 4.0.
As there are very few products that require .Net 1.1, one can wait with its installation until there is a real need for it.
No, if you never install an application that requires .NET 4, then you will never use what you've installed. As such, only install the ones you need. When you install something, as was said before, it should check and install the frameworks it needs.
In fact, if you install .NET 4 but don't use it, you'll have extra patches to download, as you'll have to patch it up to date all the time.
I never install frameworks until I have to.
I don't really understand your question. If you run an application that is built for this version, you will benefit from the new features and security patches that Microsoft put in this version.
Here is what wikipedia says about .Net Framework 4: