Using v4+v6 actually opens two PDP contexts through the provider's network. Since they pay for their equipment on a per-context basis, they would rather you have only one at a time. I was getting kicked off regularly when I had v4+v6 setup, but I'm not sure if it was some automated process on tmobile's side or something else. I'd be interested to hear how everyone else's connection reliability fairs when using v4+v6.