iPhone 4's had terrible problems with cell signal. It's usually not caused by the antenna, unless it's simply disconnected or got broken during another repair (which actually happens quite a bit because of where the antenna is located). The antenna is essentially just a wire, so it doesn't wear out or anything.
Here is the procedure I usually check when I get this issue:
1. Is the date set correctly? If the iPhone discharges completely the date will be lost and you won't be able to connect to the network until you fix it manually.
2. Are all of the grounds making good connection? Inside the iPhone there are a number of ground connections to the frame/case that need to be good or you'll get signal issues and possibly battery drain issues. The most common problem spot is the one by the battery connector.
3. Is the antenna unplugged? Sounds silly but it happens. The antenna sits atop the speaker on the bottom of the iphone. The connector is next to the battery connector. So once you disconnect the battery you'll see it. Here's a pic.
4. Is the antenna connector broken? Did the antenna click into place? If not, the connector on the board may be bad. If you have a microscope or magnifying glass you can also inspect the connector to see if it looks OK. Rarely it may need to be resoldered
5. Is the blue inductor coil missing or damaged? Google iPhone 4 blue inductor coil and click images to see what I'm talking about. It's a very, very, very common problem on iPhone 4. The "proper" solution is to replace the part, but most people just connect a jumper instead. If you don't have a microscope and a good soldering station it's not easy to do. However, some people on here have reported success using conductive pens. I always solder but I don't see why a conductive pen wouldn't work.
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Searching Or No Service On Cell Phone Quick Fix
https://www.youtube.com/watch?v=zfwPyoAz...
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