If there's no requirement (e.g. compliance, security, ...) and the features of the current version as well as the guest OS support is sufficient, and the environment (storage, backup, ...) is still supported, then it's indeed hard to find good arguments. I do have customers who are running 4.x environments with more than 1,000 days uptime and are happy with it.
One reason to consider upgrading may be the End of General Support for version 4.x on May 21, 2014 (see https://www.vmware.com/files/pdf/support/Product-Lifecycle-Matrix.pdf)
André