Can't speak to xy4, but xy3 has been as stable under the current version as the last. There *is* a sequence which crashes it -- a combination of my own peculiar key assignment and perhaps a finger flub -- but I haven't been able to identify it (produces "can't scroll define, etc" error message and crashes). But not a daily occurrence.
I'm puzzled by Carl's not finding any difference in speed difference between the two versions, because the improvement is unmistakable here. The again, at the vDos site, some of the discussion suggests widely different results on different machines versus NTVDM, so maybe there are other explanations on a machine by machine basis.