If you already have the .net framework 4 installed, you do not need it.
No harm in getting the one with it. If you need it, it will install it from that package.
It looks to just have .net 4 support, I do not even think you NEED it. Chances are you already have .net 4 installed anyway.
You can check in the add/remove programs to see if you have it or not. It was released last year, so good chance you have it already.