The Spartan-6 FPGA MCB will not support x4 memory components until MIG 3.3 which will be released in IDS 11.4.
Solution
IDS 11.3 (MIG 3.2) cannot support x4 memories because of incorrect programming of the IOIs associated with the MCB in the x4 configuration. IDS 11.4 (MIG 3.3) will properly program the IOIs for a x4 memory interface and so will be the first release that supports x4 memories.