Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
The iPhone 4s Still Has Problems With Death Grip –
A lot of iPhone 4’s struggled with a problem known as the death grip. When holding an iPhone 4 in a certain way ( demonstrated in the video ) you would start to loose signal. Apple were quick to address the situation and give customers a free bumper case which stopped this from happening. When Apple released the iPhone 4s in October 2011 they were sure they had fixed the death grip problem in the new device and first test proved this. The iPhone 4s used in the video above was in the first batch to ship to the UK so I thought it might of just been a one off but I was proved wrong when I tried it on multiple other iPhone 4s’.