From the logs of your connection to server 4.2.21, port 4000 is somehow being filtered or blocked. Port 22 (SSH) is open, so you have two options:
1) Check your firewall settings and allow connections via NX to port 4000
2) Or you can connect via SSH instead of NX provided you have a version that supports SSH (any of the Enterprise Server/Terminal Server products).
I have removed your logs of the 3.5.0 server. As I mentioned earlier, our traditional support channels are available for 3.5.0 users. If you have a subscription, log in and submit your enquiry there.