We generally continue to support earlier versions of the software for a period of 2 years, after which we can't guarantee backward compatibility of  servers and clients of major versions released later.
 

Compatibility between NX 3.5.0 and NoMachine 4

Consult https://www.nomachine.com/AR10K00719 for more details.

More information about compatibility and upgrading to version 4 is available here:

http://www.nomachine.com/AR10I00603

Compatibility between other older versions

Users will be able to continue to user 2.x.x client software with the 3.x.x server software. Likewise, NX Server versions 2.x.x are compatible with NX Client version 3.x.x.

NX 1.4.0, NX 1.5.0 are  no longer supported by NoMachine.