My site has rolled over to V4alpha and says I'm an enrolled beta tester!

I have a site that until recently was on v3.9.1 (I recall).

Unexplainably, a few days ago, it changed to 4.0 which is the beta version. Today my images have stopped working for new posts seemingly due to a permissions problem that I cannot resolve.

My installation tells me I’m a beta tester and that I’m set up for nightly updates – wtf is going on?

I need some help – this is my production machine!