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.
Foot:
Sensormatic free motion foot
I had trouble putting on the foot at first. As shown below the top plastic hooky thing goes in front of the needle bar.
Feed Dogs:
Lower the feed dogs by pushing the feed dog lever on the front of the machine to the left.
Settings:
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.
Thread:
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.
Needle:
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!!