The 4.0 version of the secure, anonymized, private browser disables SSL3 (in deference to the POODLE attack) and uses new transports that are intended to defeat the Great Firewall of China and other extremely restrictive firewalls.
More importantly for censored users who were using 3.6, the 4.0 series also features the addition of three versions of the meek pluggable transport. In fact, we believe that both meek-amazon and meek-azure will work in China today, without the need to obtain bridge addresses. Note though that we still need to improve meek's performance to match other transports, though. so adjust your expectations accordingly. See tickets #12428, #12778, and #12857 for details.
This release also features an in-browser updater, and a completely reorganized bundle directory structure to make this updater possible. This means that simply extracting a 4.0 Tor Browser over a 3.6.6 Tor Browser will not work. Please also be aware that the security of the updater depends on the specific CA that issued the www.torproject.org HTTPS certificate (Digicert), and so it still must be activated manually through the Help ("?") "about browser" menu option. Very soon, we will support both strong HTTPS site-specific certificate pinning (ticket #11955) and update package signatures (ticket #13379). Until then, we do not recommend using this updater if you need stronger security and normally verify GPG signatures.