Thats the point if you have oddy you cant play in Horizons 4.0 DOH!
As Bruce said, H4 is basically an "Odyssey Demo". The idea is if you like H4, spend a few bucks and upgrade to Odyssey. On sale the cost is cheaper than one month of a subscription-based MMO. If they don't like it, they'll go back to H3.8.
I honestly don't see the problem here. Bruce has confirmed one of my theories (and disproved my opposing theory) - Horizons 3.8 remains the de facto Horizons. H4 is kinda like shareware, you can try it, but if you want all the features, you've got to buy it. That seems fair to me. Frontier just has to be really careful with how they market both Horizons and the Odyssey DLC, because most games don't have DLC that separate the player base the way Odyssey does (buying an ESO DLC doesn't prevent me from playing with those who haven't).