I have the 4120 QDC and the Quilt Expression 4.0 and both have great features. The 4120 QDC is for traveling to classes, retreats, etc. and I use the Quilt Expression at home.
The 4120 is lightweight, portable, and user friendly. It sews smoothly without the machine jumping because of the speed. The thread cutter, needle up-down is nice too. The only short-fall is that the harp space is a little small compared to the Quilt Expression.
The Quilt Expression is absolutely wonderful. I just purchased mine about a month ago, and have had no problems. I got mine used, and I couldn't be happier. It was a bit of a learning curve going from a Janome MC 6300 to the Pfaff, but man are those features nice. The thread cutter, needle up-down are standard for this machine, but I especially like the automatic foot raiser for pivoting on corners and after the thread cutter is enabled. The only thing I don't like is that the guide for the 1/4 inch foot is silver, which reflects the fabric I'm sewing and makes it a little difficult to see how accurate I am in my piecing, whereas the Janome 1/4 inch foot has a black guide on the right side.