I haven't bought a V5 yet or even opened a V5 project in the Lokprogrammer software so I'm a little in the dark on the details. I didn't know that Drive Hold, Coast And Run 8 were now logical functions as opposed to sound slots in the file. Hopefully this means any V5 project can use these features. This is good news, as I was worried that the upcoming MM decoders for the 121 locos would not have these features. As you say, those features aren't really of interest to the European market.
As regards V5 vs V4 compatibility; V4 projects will work in V5 decoders. Now whether you need to use the sound slots to implement Drive hold/Coast/Run 8 or logic functions I really don't know.
V5 projects however will NOT work in V4 decoders.