Calcutta is a codename for the TeamCity 4 version.
This version of TeamCity contains new features, improvements and multiple bug fixes comparing to TeamCity 4.0.2.
Please back up your data (database and .BuildServer directory, backup explained) before upgrade!
Build #8725 contains bug: it can mix up your artifacts. Please apply this patch after the installing this build: http://www.jetbrains.net/devnet/message/5232624#5232624