Your setup is basically going to be the same. However, you do want to make sure the virtual IP address on both WLCs on the 2nd WISM you purchased are configured the same as the other WISM if you intend to allow mobility between APs and clients. Additionally, you will also have to add your two mobility members to your existing controllers and configure them correctly on your new ones.
If you intended to configure manual load balancing you can use the primary/secondary/tert configuration on the access points.
All of the above is mentioned in the 4.0 configuration guide and it does cover multiple WLC implementation as well:
We are moving! Please use WLCCA Forum for updates and discussions
[toc:faq] Wireless LAN Controller (WLC) Config Analyzer Download Click
here to Download To request access, send an e-mail to
email@example.com. Please include your Cisco.com userna...
[toc:faq] IntroductionHere is the step by step process that we have to
take care of while converting LWAPP to IOS and then vice versa..LWAPP to
IOSThe hardware used = 1141 AP (make sure we are using the right
[toc:faq] Introduction AnyConnect Secure Mobility Client 3.0: Network
Access Manager & Profile Editor on Windows Summary Use the Cisco
AnyConnect Network Access Manager Profile Editor to build custom
profiles for the AnyConnect Secure Mobility Client. App...