Does .NET Framework 4.0 require restart?

It can require a restart.

The .NET Framework setup will require a reboot when one of the files it needs to install is in use during installation. There are a handful of files that are shared by all versions of the .NET Framework, so if there is an earlier version of the .NET Framework installed on the system than the one that is currently being installed, and a managed application is running and holding one of the shared files in use, then a reboot could be required at the end of installation.

More details here.

If a reboot is needed, you'll find two entries in System event viewer, like:

Event id: 4376

Servicing has required reboot to complete the operation of setting package KB956250(Update) into Install Requested(Install Requested) state

and

Event id: 4386

Windows Servicing required reboot to complete the process of changing update NetFx4x86 from package KB956250(Update) into Install Requested(Install Requested) state

It depends.... It depends on how up to date the computer is, what else is installed. There are too many factors to actually know for certain. My experience is we have installed it on 7 machines both 32 and 64 bit running Server 2003 & 2007 some required a reboot and others did not.