Why does my myTouch 4G lose battery so fast?
You can install BetterBatteryStats (from Play Store or from XDA, the latter is free) and see which is causing this.

If you're running the phone without a SIM card this might also be the reason:

Some phones aren't tested well if there's no SIM inserted (my old Xperia X8 is such a phone for example).

Narrow it down this way:  


Switch the phone to airplane mode (switches wifi/bluetooth/gsm off)
Then re-enable wifi (plus maybe bluetooth) again. Just leave it for a while and see if it's better now.