Well from Samsung Singapore's facebook page they said...
JG4 should have:
1. Includes new Task Manger which works like Advanced Task Killer App on Android Market
2. Solves the World Clock lock issue
3. Solves the Kies connection problem
4. Includes new tutorial of DioPen Chinese IME mode
5. Solves unable to send MMS in 2G mode issue
6. Solves Outlook sync issue
7. Changes Silent mode icon to GPS icon under the pull down menu bar
Now if only we can get a full JG4 Rom cause the one on samsung-firmware is broken..