Present a new CX4 LUN to the cluster that will be the new quorum disk.
This means that you will have all clustered hosts zoned, registered and in a Storage group on the new CX4. You add the one new LUN that will be the new quorum and configure as a physical disk resource.
Follow the Microsoft KB article http://support.microsoft.com/kb/280353 for instruction on how to move the quorum to a new disk. You do not need to migrate any of the data from the quorum. Hopefully you have setup the quorum as a dedicated LUN, if not then you will need to migrate user data after you move the quorum. Make sure you are able to start the cluster successfully and failover and back before going any further.
Next, I recomend using EMC SAN Copy to migrate the other cluster disks from the CX3 to the CX4. It will copy the disk signatures. Also make note of any dependancies you may have on these disks.
Shut down the cluster
Setup SAN Copy (source to target)
SAN Copy the data
Remove the clustered hosts from the CX3 Storage Group
Add the new CX4 target LUNs to the new cluster Storage group on the CX4. Add the LUNs in the same HLU order that they were in the CX3 Storage Group.
Start up one node of the cluster then the others.
My apologies, I just realized that you have Windows 2008. The instructions above are for Windows 2000 and 2003. I have not actually moved a Windows 2008 cluster. I would recommend taking a look at the Microsoft KB http://support.microsoft.com/kb/957256. I believe that you will need to export the configuration. See the section on Exporting and importing virtual machines in clustered environments.
You should still be able to use SAN Copy to move the data from the CX3 to the CX4 LUNs.
Again, I have not actually done this for Windows 2008 so hopefully someone with real world experience will add some insight.