What's the difference between and Iphone 4 and a Iphone 3gs?

Like I'm probably getting an iPhone 3gs, but I want to know, can you do everything you can do with an Iphone 4 on an Iphone 3gs?

The 3GS only supports up to iOS 6.1.6 meaning you can't install the newest version of iOS. This also means you are limited on apps, since some apps require iOS 7 and higher.
Apps aside, you should be able to do anything you can do with an iPhone 4 on a 3GS.

One is newer

The iphone 4 can video call such as FaceTime while the 3GS can't

iPhone 4 has a better camera - The camera on the iPhone 4 is a 5 megapixel camera while the 3GS has a 3.2MPX camera

iPhone 4 has better resolution

The iPhone 4 has more memory than the 3GS does