kind of depends how much your willing to spend, and how much you like having a auto, and your warranty...
apio gearsets give 58% in low only, but will set you back over 2k AUD.
http://translate.google.com/translate?d ... o/6020_01/cant help you on unlocking 4th sorry.
theirs kits for larger cars that lock up the torque converter much tighter, making them both more efficient/responsive while limiting overun, however from my experience with the manual, theres still a bit of overun with such a small engine.
for the costs involved, routinely replacing your break pads and cycling your peddel pressure to prevent overheating the breaks would be the best bet.