How to upgrade the login shell from bash version 3 to 4

Apple will not update Bash, because the latest version is licensed under GPLv3, which Apple cannot use. They have updated most of their other shells though. ZSH for example is mostly up to date.

References:

After a bit of research, this seems like the primary issue:

• https://www.gnu.org/licenses/gpl-faq.html#Tivoization

When people distribute User Products that include software under GPLv3, section 6 requires that they provide you with information necessary to modify that software. User Products is a term specially defined in the license; examples of User Products include portable music players, digital video recorders, and home security systems.

This would require that otherwise closed-source software, have its GPL'd portions be made modifiable by the public, which would obviously be an issue for Apple.

You can use the chsh command to change your shell. In your case, with brew generated bash, you should set it to /usr/local/bin/bash. You should also put this in /etc/shells.

As to your question echo $BASH_VERSION will tell you the version of the current shell. bash --version tells you the version of the first bash in your $PATH. So the way you were looking at the version was not telling you the version that you were running.