First I have to make this warning. Opening an iPhone can cancel your warranty. If you have a warranty just take your iPhone in to be fixed or replaced. Having said that. Here we go.
My iPhone 4 has had terrible wifi reception. I basically have to be in the room with the router for it to work. I searched on line and found a number of ways to fix this. Most are very technical.
But I decided to try the simplest method and it worked! I now have great wifi reception. This solution won't fix all iPhone wifi antenna issues. But it's the simplest so it's worth giving a try.
First. Turn your iPhone off.
Some iPhone 4's do use tiny Philips screws. So check to see if yours uses Philips or a pentalobe screwdriver. It's crucial to use the correct tool!
Put the iPhone back together and turn it on. See if the wifi improves. If it does then this simple fix worked.
If it didn't fix it then I suggest you google "iPhone 4 wifi antenna fix" for more possible solutions. Some are very technical with tiny parts. So be careful. Or find a professional to help.
But as I said. This was simple and it solved my problem. So I figured it was worth sharing. Good luck.