Compatibility between v3 and v4
Note: NX client v3 cannot connect to NX server v4 via SSH; only via NX. SSH connections are only supported with a paid server v4 version.
Setting up a Linux to Linux connection
Installing nomachine_4.0.369_1_i686.rpm on both the server and the client works, except that on the server iptables needs to be turned off. The appropriate rule is to allow TCP port 4000 and UDP port 4722. Use system-config-firewall to add 2 custom rules. Verify with iptables -L afterward.
Also note that once the NX connection is established to the target machine, a new virtual desktop session is not established as in NoMachine v3. Rather the behavior is like Windows "Remote Desktop" which shadows the current desktop session that is already in use on the "server" machine (if it's already logged in).