I don't use Melodyne very often, but I want it to be up to date if I need it.
I read in the "SONAR 2016.12 Update" newsletter about the "Free Melodyne 4 Upgrade" back in January.
I have Sonar Platinum with lifetime membership and Command Center shows everything is up to date.
In the Cakewalk Store it shows Melodyne Essential with a date of 11/28/2013. 
I reviewed the list of "Available Downloads" and do not see Melodyne.
 
If I try to add Melodyne to a track, it pops up the license request. I insert my serial number and it says "Melodyne essential 2"
When I click continue to license it, it responds "Server Error Code 113 Product Version Not Acceptable".
 
How do I install and License the current version 4 of Melodyne?
 

timg11
I don't use Melodyne very often, but I want it to be up to date if I need it.
I read in the "SONAR 2016.12 Update" newsletter about the "Free Melodyne 4 Upgrade" back in January.
I have Sonar Platinum with lifetime membership and Command Center shows everything is up to date.
In the Cakewalk Store it shows Melodyne Essential with a date of 11/28/2013. 
I reviewed the list of "Available Downloads" and do not see Melodyne.
 
If I try to add Melodyne to a track, it pops up the license request. I insert my serial number and it says "Melodyne essential 2"
When I click continue to license it, it responds "Server Error Code 113 Product Version Not Acceptable".
 
How do I install and License the current version 4 of Melodyne?
 

I'd go directly to the Celemony website and enter your credentials and see if the new installer is available there for you.  What you are doing now appears to only be activating within the program.  Melodyne 4 will require a whole new install before that version will appear. 

I was confused by the FAQ: https://www.cakewalk.com/Products/SONAR/SONAR-FAQ#Melodyne
which says "I already have Melodyne 2 Essential from my SONAR purchase. Will I receive an update to Melodyne 4 Essential?  Yes. All you need to do is to run the “Update Check“ from within Melodyne’s preferences and it will guide you through the update process upon which you receive a new installer along with a new serial number from Celemony."
 
From the Melodyne 2 plug-in (which will not accept my serial number), then Setting / Options dialog, check for update.  It says "This Version: Melodyne Playback Mode 2.1.2. b 2 64 bit.  No Newer Version Exists"
 
You are correct about going to Celemony's site - that is how it actually works. After logging in, I was able to download the installer.  I closed Sonar, ran the installer, opened Sonar, (the VST scan identified the new plugin), then I loaded the project with Melodyne on a track.
 
The plug-in would not function or open from the existing track.
I discovered it is necessary to delete the plug in from the track, and then insert it again. At that time, Melodyne opens and must be activated.