I was using bash3 up to now, and just got a copy of the latest version
$ ./bash --version
GNU bash, version 4.1.0(1)-release (i686-pc-linux-gnu)
Copyright (C) 2009 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
$
What are the major changes/feature additions in version 4 over version 3?
The NEWS file lists the features that were added in each version.
Most notably in 4.0 I see a couple of improvements to the autocompletion infrastructure, the (optional) addition of the ** glob operator, associative arrays and various syntactic shortcuts.
autocd option — it doesn't look like much, but as a zsh user, it's what I miss most when I occasionally use bash (<4 or unconfigured).
Most importantly for the behavior of old scripts, the behavior with set -e has slightly changed, among other things for ( ...), [[ ... ]], and (( ... )).
The new behavior is said to be documented in the manual.
For example:
$ bash3 -ec '(false); echo $?'
1
$ bash4 -ec '(false); echo $?'
$
