How to unlock/reset/jailbreak an iPhone 4?

A family member sold me his iPhone 4 (unsure which model, although the software appears updated) because he got the new 5, but he failed to tell me that the phone was still locked. I don't know how to get in contact with him to figure it out, and therefore can't do the online reset I've been told about.
Is there any other way to reset, unlock, or "jailbreak" the phone so that I can actually use it, or am I pretty well screwed?

Should I call police? What do I do?

I needed a phone since my old one was broken. A kid from my school who I normally don't talk to offered to sell an iphone 4 to anyone with $50. He stated there was nothing wrong with it no cracks or damage. I paid him the $50 and got the phone. About a week later I noticed the phones speaker didn't work and not did the camera. I went to verizon to get it fixed and everything inside was missing screws, water damaged or burnt. I spend another $26 repairing it there. I asked the kid for my money back and he said he'd pay me the next Friday. Friday came and nothing. I went to message him on Facebook an he blocked me and took my money. What do I do?

Unjailbreaking iPhone 3GS custom ipsw?

Recently I decided I didn't to be jailbroken so I went to do the process on iTunes on my PC where I clicked restore and a message would appear about backing up my contents. I did and as the time passed a message came up saying my iphone couldn't be restored. I'm still jailbroken and have all my stuff but I want to unjailbreak. Note again I have an iPhone 3GS on ios 6.1.3 and I used a custom ipsw when I jailbroke.

Unlock iphone 5S from icloud activation screen?

Basically i've been given this iphone 5S that someone had bought on ebay they give it to me because they couldnt get past the icloud activation screen because the person they bought it from has just stop talking to them and won't give out the apple i.d or remove the phone from find my iphone, so i've restored the phone and i'm wondering if there's any way at all to remove the apple i.d that is currently on the phone so i can put mine onto it i've had a look around the internet and i've seen it done but i wanna be able to do it on this… Anyone got any suggestions?