Chuser,
Your question does not indicate which revision of PCD you have, but judging by the 4GB system partition being established it is an older one.
The original rationale for the 4GB usage on certain smaller servers with a single storage volume (one disk or disk array) stemmed from the need to provide as much space as possible for those systems that would eventually have Microsoft Exchange installed on them. MS's EseUtil utility requires unused disk space at 110% of the size of the Jet database it is operating on. Worst case considerations led to adopting the 4GB system partition.
8GB later was standardized to become the system partition size, as disk sizes simply grew larger as servers transitioned.
http://www.cisco.com/univercd/cc/td/doc/product/voice/c_unity/pcd/pcd_inst.htm
You can look at the above document to determine the version of image that you have. If you have a UNITY-SVR1400-1A you will probably have a PCD at revision G. If so, no later images were produced for it. Partition "stretching" utilities exist, but none have been validated for a Unity server. A 4GB does function suitably for a system partition in a machine installed to proscribed capacities. Your options include installing all binaries except the operating system (and the pagefile if you like) in another partition or rebuilding using installation commissioning SW from your HW vendor in tandem with an MS Win2k disc. Examples are Dell ServerAssistant, HP SmartStart and IBM ServerGuide.
