Hmmm. Interesting thread.
I only play Odyssey (I do use Horizons, on my laptop, to move my FCs any distance, while playing EDO on my PC) and have never been able to instance with Horizons players...
I still can't instance with Horizons players, which I was told I still wouldn't be able to.
H 4.0 gives those Horizons players a chance to 'try' the 4.xx engine without Odyssey exclusive content (and, as expected, there are tears).
No change then, I still can't play with the people I couldn't play with, that's fine by me.