does it get instagram??

Yes.

When you buy it it doesn't come with these apps installed. You have to make an apple account when you first turn on your iPod then go to the App Store (blue icon) and search for these apps and download them.

I seriously advice you not to get a 4th gen. Sure, they might be cheap, but they're obsolete and don't support the latest iOS version. You can only go up to iOS 6.1.6, which few apps support. Only Kik supports that. Instagram requires iOS 7 or higher, while Twitter requires iOS 8.

You can use the web based versions of these apps, if available, but they won't run nearly as smooth as the apps themselves.

Yes