What is the difference between an iPod touch 1g, iPod touch 2g, iPod touch 3g, and an iPod touch 4g?
The main difference between iPod Touch 1G, 2G, 3G and 4G is in their appearance and features. Look at the side view of all these phones and you will notice some of the differences. You can get some more information on Repairs Universe.
The 4g is the one that's upgraded and probably better in some ways.
The iPod touch 3g doesn't have a camera but the 4g does.
appearance, performance, and 4g has cameras
There is many upgrades from one to the other