VMware have just confirmed the upgrade path for VCDX4 holders to update their certification to VCDX5.
Current VCDX4 holders will have a 3-month window from when the VCAP5-DCD exam is released where the only requirement to update to VCDX5 will be to pass the VCAP5-DCD exam, people who do that will also be awarded VCP5.
Once that 3-month window has passed, VCDX4 holders will be required to be VCP5 AND pass the VCAP5-DCD exam to achieve VCDX5. The regular current VCP4 to VCP5 training requirement will apply, such as attending the vSphere 5: What's New training course.
Read the official text here: http://communities.vmware.com/message/2037410#2037410