Well I have had my Pfaff Quilt Expression 4.0 for about 6 weeks now and finally am comfortable with free motion quilting with the Sensormatic foot. Originally, I thought I would use the spring free motion foot but actually like this little Sensormatic foot. I had great difficulty at first and I am not happy there is so little instruction in the manual. I have searched the web and sewing forums and as usual have found several places with comments and recommendations. I found that the Pfaff Quilters Forum http://www.pfafftalk.com/chatroom/mvnforum/index was most helpful on this topic. I took advice from these sources and here is what works for me.
Sensormatic free motion foot
Lower the feed dogs by pushing the feed dog lever on the front of the machine to the left.
Under the Settings Menu (the button with the tool icon) I turn on the Sensormatic Free motion. I change the Pivot Height to -3 and I turn off the Auto foot lift. This will automatically change the thread tension to 5.2 but on my machine I need to increase it to 5.4 or 5.6 depending on the thread that I am using. This will also automatically set the stitch length to 0.0.
I have experimented with embroidery thread, King Tut Quilting Thread and Coats and Clark All Purpose thread. All have worked well as long as I have the same thread in the top and bobbin.
I am using a 90/14 Organ needle; I had been using a 75/11 and experienced thread shredding and since changing to the 90/14, all is well.
So now I am quite happy with FMQ on my QE4!!