Are your 4 monitors client- or server-side?
If they’re client-side , you can opt to display the remote desktop ‘Fullscreen’ across all monitors in the menu panel (Ctrl-Alt-0 -> Display -> Fullscreen on all screens OR click the fourth little icon from the left at the bottom of the menu).
If the 4 monitors are server side and you need to be able to switch between each of the remote displays from the client, then what you need is the following feature request:
Your question about thin clients that support NoMachine, that’s really a question for the sales team. You can contact them directly from the NoMachine website: http://www.nomachine.com/contact-request