Mine lasts for around 36 hours and sometimes 2 days.

1) I've set the brightness to 4 bars
2) Set GSM instead of 3G or Dual because 3g Service isn't available yet
3) Turn of GPS and Network Positioning, I enable them only when I need it.
4) I use Autodisconnect and Wifi Swircher. I think Wifi Switcher is a great App turning off Wifi has substancially conserve the battery. Before that I had recharge it everyday.

But still I am looking for ways to make it last 4 days as OP described