What's the difference between the iPhone 3 and 4?

I want to get an iPhone, but I'm not sure which one to get. Not interested in using it as a phone, I already have one, and I just want to buy a used one on eBay. I don't want an iPhone 5, so I suppose I want either an iPhone 3 or 4. (Not 4S) So…

What is the difference between the iPhone 3 and iPhone 4? Also, besides calling, what separates the iPod from the iPhone?

Hard to go into all the detail - DON't get a 3, get a 4/4S/5

Only calling and some other useless stuff are the differences between an ipod and an iphone. You should buy an ipod touch 4g or 5g if you don't care about calling

For the iPhone question

iPhone 4:
Front camera
Retina display
Rectangular design
Camera flash
Faster
Thinner
Apple A4 processor
Better camera
FaceTime
Better battery life

iPhone 3GS:
Cheaper
More durable

So your choice, but do not get the original or 3G version