What is the capacity of an iPod Touch 4?

It depends on your iPod. You can check the capacity of your iPod by plugging in your iPod and opening up iTunes. Then, once your iPod has synced, you can click on your iPod under the devices tab on the left. Near the picture of your iPod it will say the capacity (software is already subtracted from the overall total, so you don't have to worry about that). You can check how much space your iPod has left by looking at the colorful bar at the bottom. That part is very self-explanatory and is labeled below it.