4th cylinder spark plug stripped because it was put in right. Reason to how I found out was I was driving on the freeway and I heard a pop noise, and a loud clicking occurred after whenever I would accelerate. I looked under the hood and the spark plug shot out completely and broke the spring and everything. I took it to a mechanic and he said since the spark plug stripped the surrounding area, that he would have to take out "Heads" to get to the part to fix it. Anybody know what this means? He also gave me an estimate of 1,500$. I was told he was trying to cheat me. Any answers will be greatly appreciated, thank you.