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We're running WCS version 4.1.91 and have not been able to have a wlc running 4.2 join it. Should the WCS version be higher than that of wlc?
WCS should be higher or equal with the WLC version, not the other way.
Just to add a note to the good info from Zhenning, here is the compatibility note that you will see in all WCS Release notes :)
This release of Cisco WCS (188.8.131.52) for Windows or Linux is compatible with wireless LAN controller and Cisco Aironet lightweight access point release 184.108.40.206 or later.
Note: You may experience compatibility issues if you add a controller with a newer release than the WCS release. For example, 220.127.116.11 controller software release should not be added to WCS 4.0.
Release Notes for Cisco Wireless Control System 18.104.22.168 for Windows or Linux
Hope this helps!