VMware announced vSphere 4 on April 21, 2009 and released it on May 21, 2009. VMware released Update 1 for vSphere 4 on November 19, 2009 to add support for Windows 7 and Windows Server 2008 R2. VMware vSphere 4.1 began shipping in August, 2010, and delivers additional functionality, including an updated vCenter Configuration Manager as well as vCenter Application Discovery Manager.

Below is the differences between VMware Vsphere 4.0 and VMware Vsphere 4.1.

Figure : Courtesy VMware

So on conclusion there are two versions of Vsphere so far Vsphere 4.0 and Vsphere 4.1.