Has your 4th account already been established at the service provider's website? If so, you would add it to TB from File/New/Existing Mail Account, or Preferences/Account Settings/Account Actions/Add Mail Account. I'm not aware of a 'Create a new Account' button - can you post a picture?

Thanks, I was able to add my account earlier using the flow you suggested. Here's a screenshot of the "create a new account" zone, which I got to by clicking on any of my already-configured email addresses:

OK, on my system (W10), clicking 'Create a new account', 'Email' brings up a window with two buttons, and clicking 'Skip this and use my existing email' shows the same window as File/New/Existing Mail Account.

But I'm not sure why it doesn't work on your system, although there have been cases recently that suggest changing your system's display settings may be a remedy.

Wow, yeah, *lowering* the resolution of my screen results in those buttons working correctly. I still think this is a bug, but I'm closing as solved since there's a workaround.

Thanks for all your help!

So the place you couldn't add accounts is the so called "Account Central"? http://kb.mozillazine.org/Window_layout_-_Thunderbird#Account_central

Steve, what do you think is the cause?

Wayne Mery said

So the place you couldn't add accounts is the so called "Account Central"? http://kb.mozillazine.org/Window_layout_-_Thunderbird#Account_central Steve, what do you think is the cause?

It's possible that some TB controls don't work on high-resolution displays, e.g. 4K, and maybe it hasn't been noticed by developers working on conventional monitors.