Is your iPhone 4 not working when you make or receive call? Does it the mic still work in other apps? Have you recently dropped it? Even if you haven't try this out. Super easy to do.
What you need:
00 Philips or Pentalobe screwdriver
Piece of paper or cardboard
Scissors
iPhone
If you don't have the right screw driver borrow it from a friend. Otherwise they are pretty cheap to find, 5 USD max