In general, NoMachine Enteprise Client 4 and NX Client 3.5.0 can perfectly coexist on the same host machine.

Connection files (.nxs files)  from client 4 and session files (.nxs files) from 3.5.0 are compatible and can be used indifferently by both clients.

By default client 4 saves its connection files (.nxs) into the user's  Documents/NoMachine directory while client 3.5.0 saves its session files to the user's home/.nx/config directory.

If you would like to keep .nxs files from client 4 and client 3.5.0 into the same directory, you can change default settings for client 4: from the GUI, click on 'Change player settings' and go to the Player environment panel. You can specify path where to save connections file from the Connections field.

On Windows

NoMachine Enteprise Client 4 and NX Client 3.5.0 can perfectly coexist on a Windows host.  The first installs in the NoMachine Enterprise Client directory and the second in the NX Client for Windows.

On Linux

By default both NoMachine Enterprise Client 4 and NX Client 3.5.0 install in the /usr/NX directory.

You can make them coexist by installing client 4 (which supports relocation) into a different directory and then install client 3.5.0.

Note that for installing client 3.5.0 from rpm or deb you have to specify option to ignore dependencies. Dependencies on nxclient are necessary for upgrading installion of version 3.5.0 to version 4.

RPM

Install client 4 to for example /opt:

# rpm -ivh nomachine-enterprise-client_<pkgVersion>_<arch>.rpm --prefix /opt

Install client 3.5.0:

# rpm -ivh  nxclient-3.5.0-7.<arch>.rpm  --nodeps

DEB

Install client 4 to for example /opt:

$ sudo NX_INSTALL_PREFIX=/opt dpkg -i nomachine-enterprise-client_<pkgVersion>_<arch>.deb

Install client 3.5.0:

$ sudo dpkg -i --force-conflicts nxclient_3.5.0-7_<arch>.deb

TAR.GZ

Install client 4 to for example /opt:

$ sudo tar xvzf nomachine-enterprise-client_<pkgVersion>_<arch>.tar.gz
$ sudo NX_INSTALL_PREFIX=/opt /usr/NX/nxclient --install

Install client 3.5.0:

$ cd /usr
$ sudo tar zxvf nxclient-3.5.0-7.<arch>.tar.gz

On Mac OS X

NoMachine Client 4 and NX Client 3.5.0 can coexist on a less recent Mac OS X  up to version 10.6.

NX Client 3.5.0 ships PPC-based GUIs  that require the Rosetta translator for working on an Intel-based Mac:  http://www.nomachine.com/AR05I00593

Support for PowerPC applications is no longer available since Mac 10.7 Lion.