I just downloaded the trial for Melodyne 4, and it
does overwrite Melodyne 2 (can see the "updating Melodyne 2.xxx to Melodyne 4.xxx"). The code is identical for all flavors (Essential, Assistant, Editor, Studio), so I can only assume that Melodyne 4 Essential will overwrite Melodyne 2 Editor.
The demo itself has an issue with activation, as it goes to a web page for "trial" but remained in "Media Player" mode (i.e. opens to look like Melodyne Studio, but cannot edit anything).
I just sent an email to Celemony on both issues above and will let you know feedback I get. As of right now, I have a non-functioning trial and total loss of Melodyne 2 Editor.