After the upgrade to NoMachine v8 it's no longer possible to upgrade the port for connections by NX protocol (nxd port 4000) via User Interface.

This is because the correspondent configuration key has been renamed in v8 but it's not present in the server.cfg file v7.

As a workaround, add the missing key to the end of the current server.cfg file:

NXTCPPort 4000

and set it to the wanted value, e.g.

NXTCPPort 4100

Other keys v8 that could be affected as well are:

NXTCPUPnPPort ""
NXUDPUPnPPort ""
SSHUPnPPort ""
HTTPUPnPPort ""
UDPPort 4011-4999
SSHPort 22

v7 uses:
NXUPnPPort ""
SSHDUPnPPort ""
HTTPUPnPPort ""
UDPPort 4011-4999
SSHDPort 22