How do I switch carriers with an iPhone 4?

So a few months ago I had my iPhone 5 stolen. I bought a 4, trying to cheaply replace said 5. However, upon receiving it and trying to put my SIM card into it - and consulting these forums - I see that the 4 is slaved to its carrier. It uses CDMA, not GSM, and basically won't even work outside the US. All this would be fine, except I live in Germany.

So, ladies and gentlemen, please help me explore my options. I'm writing these out as I think as I go through everything I'd do if I was still in the US.

Obviously I could return the phone (I bought it off of Amazon) and be out of an iPhone for another month (two weeks to send this phone back and get a refund, two weeks to receive the 4s sitting in my shopping cart).

As I'm in Germany, I have no idea if the companies here even take phones on trade-in, let alone ones assigned to American carriers. If it's possible, I'd love to trade this in for a 4S and pay the price difference, but again, do German phone carriers even do such a thing? Especially if the phone is from the US?

Speaking of trading in or even selling the phone, would it even be worth it to try to sell it over here as it's a Verizon phone?

Also, speaking of other carriers, is there any way I can....basically turn this Verizon iPhone into, say, a Vodafone or O2 iPhone? (I've had Verizon for ten years and never had to deal with these issues, pardon my ignorance).

Help me out, people, I know we have Apple users from all over the world, so if you had an iPhone 4 that is from a US carrier, but you did not live in the US, what would you do?

Thanks. It's still a 32GB, so I'll just use it as a glorified iPod until I figure out what to do with it. It's still half the price of a brand new iPod Touch, so...there's that, I guess (forced optimism ftw)?