I think this question would be more valid and acceptable for Server Fault if you explain the licensing problem you're trying to solve in greater detail.
The Intel X5675 is a 3.06GHz 6-core CPU. I'm assuming you're using two CPU's per system. Depending on the hardware you're running, it is possible to reduce disable half of the cores on the CPU in the BIOS. What type of servers are these? Dell? HP?
Your 4-core options for that CPU generation are the Intel X5687 or the Intel X5672, which are 3.6GHz and 3.2GHz, respectively. You can also use a handful of slower CPUs, like the E5620, but that's a step down.
Oracle is popular and has good mindshare here. I have not heard of anyone needing to go to 4-core CPUs to fix licensing problems.
Please also see my answer at: Disable CPU cores in bios?