I have concluded that Skype & Multitasking to are 100% blame for the poor power consumption of my new iPod Touch 4g vs. my older iPod Touch 2g which did not have this problem. The problem is NOT related to wifi!
On the 2g iPod Touch, the moment you leave Skype (or any other application), that application will automatically quit. Thanks to the multitasking provided on the new iPod Touch 4g, Skype and other applications stay OPEN, even after you leave the application.
The advantage of this is that I can now receive Skype calls on my iPod Touch 4g, even when I am not in the Skype app, and even if my iPod Touch 4g is 'sleeping'. This is very handy when I am on a trip, and am expecting call (and don't want to leave my iPod Touch in the Skype app all the time). The BIG disadvantage of this is that Skype will suck up the entire battery in less than a day.
Thankfully, there is a way to quit Skype and any other open applications when you don't need them. If you double-click the home button, a "tool bar" will appear at the bottom of the screen, which shows icons for all of the applications you recently opened, that may or may not still be open. If you press and hold any of the app icons for a few seconds, a small white "Minus symbol" will appear in a small red circle on the upper left corner of each of the app icons, and the icons will start to 'jiggle'. Clicking on the minus sign of any app icon will close (force quit) that app.
Now that I don't leave Skype running in the background all the time, my iPod Touch 4g lasts through several days of moderate use without requiring a charge, even with wifi and push notifications turned on. (I have my mail preferences set to check my gmail every hour)
Problem solved! Skype should give users the option in the preferences of always of running in the background (multi-tasking), or automatically quitting when you leave the application, but in the meantime, this is an effective (if clumsy) way to quit Skype when it is not needed. It would be nice to be able to leave it running all the time, but it just uses too much power.
TO APPLE: Please put a MUCH larger battery in the iPod Touch 5g so we can effectively utilize features like multitasking with Skype and other applications! (Also a 5MP rear-facing camera WITH an LED flash would be nice too!) I am willing to sacrifice a little bit of 'thinness' for functionality.