Mr. Steele,
The avatar is being replaced with the default avatar by the "User - Welcome Message" plugin. Not sure what that is, but when I disable it, my avatar isn't erased. I took a quick look at the code for that, and honestly can't see where it's resetting the avatar, but it must be in there somewhere.
If you have more info about the plugin (if you absolutely need it to work on your site), let us know it, and we can investigate more. If not, I'd suggest just leaving it disabled.
As for why you're seeing this in 4.0, it's because we changed the login flow to be much more compatible with a lot of plugins and Joomla-right in many ways. The registration flow was a little odd before for when we triggered the User plugins to do their thing, and it looks like the Welcome Message plugin is a casualty of this change.
Let us know how your testing goes, or if you have any other info/thoughts.
Thanks,
Alex