Will there be a start button on the bezel on the SP4? I think no
I would happily see it go myself. Windows 8 on my surface 3 and windows 10 on my surface pro 3 and never used the start button once. In fact it just annoys me, have to rotate the tablet in various orientations depending on the usage to stop me accidentally pressing it. I have no reason to believe they will remove it however. Who knows, maybe they'll give it some fancy workover, be it fingerprint or cortana etc. I believe it is lenovo who are going to be including a dedicated cortana button on certain windows 10 products.