What vessel should/can I use to brine a bone-in turkey breast? I cannot find (in stores) a container (like ATK); shld I use a bag or (covered?) bowl?

A brining bag is a great way to go if you have time to pick one up (they have them at Whole Foods and some other grocery stores). If you don't have time for that, you can double up on gallon-sized freezer bags (if your turkey breast isn't too big), or even use a pot with a tight-fitting lid!

You could also do plastic wrap and aluminum foil like in this recipe! https://food52.com/blog/11723-torrisi-s-turkey

I bought a 7 lb turkey breast and used a 2 gallon Ziploc to dry brine it. My store also carries brining bags, but they were a bit too expensive for my taste. A 4 lb breast may have fit in a one gallon Ziploc.