When you replace an iPhone 4 screen you are required to take out the antenna bridge assembly (the part that gives signal to your phone).

Below is an image of the antenna bridge assembly by itself. Circled in red are the four metal feet that help with overall WiFi and phone reception, if any of these are broken or missing (which is easy to do as they are very fragile) then it will impact on the overall reception.

Below is an image of the antenna bridge assembly inside the actual chassis of the iPhone. You can see why the feet are so important as they connect to the main chassis and use it as the antenna.

Below are two images of one of the two screws that secure the assembly into the chassis. This screw placement is more important as the other as it also connects with the chassis as the antenna.

Below is an image of the actual antenna cable connected to the logic board. This is the MAIN connection and if this is not securely connected then you will get very minimal GSM reception.

If all of these parts of the antenna bridge assembly are connected properly and not damaged, then you will most likely need to replace the antenna bridge. HOWEVER before you do that you should carry out a full firmware restore/hard reset (disconnecting the battery for several seconds). My guess is that you have been unaware of these small parts and have accidentally damaged them through repair process (i did exactly the same thing when i first started fixing i4's)

See how that goes and we will go from there :)

Try resetting the phone

Hold home and power together

Connect the small pop-off Cap connector that is next to the battery connector. Try reseating it on one iPhone, see if it fixes it.

i watched the battery replacement and that woman said if you touch the battery slot cable itll weaken the signal cause of our natural oil on our fingers so try cleaning it out

thanks alot for the tip. this is my first time to replace the screen. had the same problem. thanks to your tip i got the phone fixed.

So I checked everything and even cleaned it all with some iso but still no luck...

I read somewhere about screws being too loose or tight can affect the signal. So how do I find a happy medium for tightness?

I think the dock itself is fine bc it worked perfectly before, so this Must have been something I did or didn't do.

Any help would be greatly appreciated, I will exhaust every option!

P.S.

I was sure to put every screw back in the same place it came from. Telus told me to check the shields in case I didn't put it in properly.. Is there any particular position it should be in? I know it's facing the right way but maybe it should be a little more left or something..?

You can see my post too. I have a weak signal problem when I enable the 3G. Is your problem same? Is it stronger when the 3G is diabled?