The four singers in question are Bono, previously Paul David Hewson; Pink, also known as Alecia Beth Moore; Enya, whose full name is Enya Patricia Brennan, and Eminem, who took his name from his initials (Marshall Bruce Mathers III). Is it a requirement to have three listed names like this? I was surprised to come across it via Wikipedia.
Other singers known by a single name? Why, I'm glad you asked. I came up with Sting and Cher, each of whom would be very difficult to fit into this theme pattern. Others that come to mind are Madonna and Prince. I would in general be loath to accept individuals who simply are known by their first name, like Elvis or Beyoncé. Yes, they are super celebrities, but it's a little like saying that RAFA is a mononym.
Anyway, I enjoyed working through the theme answers to discover the hidden gems. Once again, I'll note that it would have been better without the circled letters, but that's a losing battle.
I like how 4D (SITBACK (and relax)) is symmetric to BADMOVE! Doesn't it make you feel like there's a story going on here? Somehow, the last several words in the SE imply a danger NEARS, as we VEER, or somebody ERRS.
Perhaps. So let's end with my favorite Tuscan city, SIENA. I highly recommend visiting.