Separate names with a comma.
I don't know about Terraria, but the Junk Jack X beta ran fine on my old iPhone 3GS, so you'll have no problem running it.
It works flawlessly on iPhone 3GS, so I'd think that it'd work fine on an iPod 4G.
The backup feature has been removed, so no duplication glitch. However, they did add creative mode so you have nothing to worry about.