Hi Tusler.
New Horizons IS V1.4.
However, in another thread Bottman acknowledges that there some issues with a couple of the Padstow missions. These were caused by some last minute improvements to the harbour which then adversely affected those missions. They don't seem to affect everyone, but if you approach a couple of waypoints from "unusual" angles you might miss a hidden trigger.
Other than that, I'm not aware of any mission related problems. I don't know from practical experience if all the missions work OK, as I haven't checked many of them.
I am busy writing new ones (just finished testing 8 new Solent-based missions, all tested on V1.3 and 1.4) which takes a fair bit of time, plus some other stuff.