In the free 4 we removed the distinction between client and server, especially because NoMachine is now available as server on all platforms. Like a Skype install can be used to both connect and accept connections from Skype clients, so can NoMachine. You can obviously disable the server and use only the client. In the past, we received lot of critiques because installing NoMachine on a host required installing multiple packages, so this change should be for the better.
Client-only packages are still available in the Enterprise downloads:
Note that these client products are still free, as they were in NX 3.