Multi-tracking and Multitrack Note Editing – See and edit multiple tracks simultaneouslySound Editor – Change the timbre by adjusting the balance between partials
Quantize to reference track – Carry over the timing of one track to another
for me the Sound Editor is worth the upgrade, especially for creative sound design. Here's some info on it from the Melodyne website:
'With the Sound Editor in Melodyne 4 studio, you can edit the sound of your recordings – just like your music – in a more musical, detailed and inspiring way. From the subtle enhancement of envelopes and timbres to the manipulation of overtones, resynthesis and radical experiments in sound design.Melodyne recognizes the notes and their overtones – also a decisive advantage when it comes to sound processing. While conventional equalizers only act upon fixed frequencies, Melodyne adapts to the audio material itself. Adjust the level of individual overtones within each track, shape the basic sound of each instrument, relate the timbre or envelope of each note to its pitch, bring out the character of each sound source.
The results are novel acoustically, often unique, invariably invigorating – yet the Sound Editor is intuitive and lends itself easily to creative use."
For more, check out the Sound Editor video:
https://www.youtube.com/watch?v=Ol0OZ3xzsjs