I have a 4.0, I bought it 3 years ago. I had nothing but problems with it. I took it back at least 12 times each time no difference.
Finally I was told to take a class to learn how to use it. While taking the class several problems came up and they fixed them. They replaced the bobbin case and adjusted several things.
Finally it is working OK. That unless you try to FMQ then it leaves loopes.
I am not happy at all with it but at least it works most of the time.