Update bash to version 4.0 on OSX
Is it possible to actually update bash to version 4.0 in OSX Yosemite? Yes.


Download / Install homebrew http://brew.sh/
by running this command in terminal.

ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"

Quit and reopen your terminal. then type 

brew install bash

Change the default shell via the terminal gui with the literal path of your new bash (EDIT: I have yet to find a CLI way that works )




Is it possible to actually update bash to Apple’s provided bash version 4.0 in OSX Yosemite?

EDIT: No. Not in the way the op is asking. E.g upgrade the current install by way of replacing itself.  It has been noted in other answers that Apple has not updated bash due to licensing issues. However downloading an updated and separate version of bash and using it as your default shell is the canonical solution, for most interpreters. Take python for example. You do not upgrade 2.7 to 3.5 you download a separate version and change your default.