I have an iPod touch 4th generation, it is a sleek device without a case on it. The back is shiny, which is just going to cause scratches. It is an okay device it has 256 mb of ram. So if you have a different device you'll notice the speed difference. I say the only reason you would get it is to play music on it, and then again you can buy a cheap music player device for that. I would recommend not buying this.
If you want to buy an iPod then I would suggest getting the 5th generation, as that is much better than the 4th. There are two versions of the 5th gen. The cheaper and expensive one, the cheaper one has no back camera and a few other features to make it cheap. While the full priced one has all the hardware. Also you asked about siri, to be honest you won't ever use it. As I have an iPad and an iPhone, and I never use Siri. iPod touch 5th generation has Siri in it, if you do want it.
So my two cents, if you want to buy an iPod buy the 5th generation, the expensive one. If you want something to just play music, you can just get an iPod nano or even a shuffle. But then again you probably have a phone for that.
If you don't mind me asking, what are you doing to do with the iPod?