i have had issues with my Pfaff 4 and the thread cutting mechanism. Apparently, there is also a little hook on the right hand side of the mechanism that interacts with the bobbin case. if the left hand hook isn't completely retracted to the left and the right hand side in it's correct position, it catches and makes it so the thread gets caught or the needle breaks. Once i saw that, it was easy enough to clean out the bobbin area. make sure cutter was all the way to left and then re-assemble bobbin area and sew away. Until then i spend one evening being so frustrated that the thread kept breaking or the needle kept hanging up.