I had a QE4.0 and it was C**P. The later versions may be better, but after I'd had 2 exchanges - one from the dealer and another from VSM after I complained to the MD, I got rid of it and bought a Bernina. It (the Pfaff) was a real disappointment compared to my old Pfaff tiptronic (which I still have).
The QE4.0 would sew in a straight line beautifully, but I wanted it for FMQ and despite trying all sorts of threads etc, it would break the thread every few inches. I got other people to try it in case it was just me, and the same thing happened to them. Also the tension hook (the uppy-downy bit) on the first one was just that - a hook, and unthreaded itself frequently. The later 4.0s had a clip fitted to reduce this possibility.
If you're looking at one, check which version it is. Or get a Bernina.