Hello Greg when i buy any used gun i check it out really good. First thing i check is serial numbers on frame ,barrel and slide make sure they match. If the frame has the letter G and no other letters just a series of numbers after that means its a replacement frame which is not bad but if its not a replacement frame i wouldnt touch it. If they have an indoor range insist they let you fire the weapon as long as you buy the ammo and pay for the range. Ive never thought about the gen 4 as i have three glocks all gen 3 and they suit me just fine i feel no need in going out and getting a gen 4. Hope this helps im sure other members will have more input on thread and gives you even more info. Jhn