The NoMachine Player GUI no longer allows you to change the font server because X11 core fonts are no longer needed for NX 4.0.

However, the Player still supports the following settings for connecting to NX Server 3.x:

<option key="Use font server" value="true" />
<option key="Font server host" value="hostname" />
<option key="Font server port" value="7100" />

You can edit these keys in the session file on the Player side (.nxs file) via an editor or via a NX Client 3.5 installation and then transfer to your machine running NoMachine Player.