You have done one of two things wrong. Either you have had this tiny part fall off and you did not know where to put it.
It goes here
Or you did not use the correct length screw in this hole. The longest screw within the iPhone goes here.
Also the oil from your fingers can affect Wi-Fi signal strength. So clean all contact points with Isopropyl alcohol.
To show you which screw is in fact correct I have edited an image of the whole set of screws within an iPhone 4.
Complete original set of screws.
Shown here is the screw that I originally said was the correct screw. In fact I was wrong.
Here is the actual screw required. Please note that it has a smaller head diameter.