There is no antenna cable on the iPhone 4; the outer metal band acts as the antenna. The "antenna" is connected to the logic board through the mounting screws!
Check the mounting screws, especially the ones near the battery connector (that's where the GSM radio is). If any are missing, replace them; if not, you might try tightening them if they are loose ... or loosening them if they are tight! (The tightness of the screws can slightly change the tuning of the circuit.)
Does WiFi / Bluetooth work?