I think Daystate have been "pushing the envelope" with regard to gun design for many years, and they can properly be described as world-leading, state of the art etc. I can't comment about the Mk4, but for every Mk3 owner with bad experience there's 50 with good experiences - but they just don't complain about their good experiences.
However, I know of a couple of "happy" Mk3 owners who bought Mk4's and who regretted it, preferring the Mk3. Not because the Mk4 was better or more relaible as such, they just preferred the whole experience.
I've had the problem of pellets rolling back into the action too, but I've had it on a Mk3, a Huntsman and a Phoenix, so the answer to that is just to take a bit of care and learn the lesson.
I can't speak with any personal experience about the Mk4, but their are a lot of people using them very happily. Personally, I love the inert feel of my Mk3.