There have been a few corner cases where Cisco has sanctioned up to 6GB of memory in a CUCM server; however, the general answer is that the MCS server specs are exact match, not minimum spec. The Supported Servers for Releases of CUCM document has a somewhat infamous footnote (2) which explains when disk or memory upgrades are required. You should follow this exactly as-is unless you have a written support deviation from Cisco allowing you to do something else.