It's a widely held misconception that all screens used in this industry are of the same quality and grade. Where in fact there is a very wide range of quality used by repairers.
There seems to be a vast range of warranty given on mobile phone repairs and tablet / iPad repairs. From as little as 30 days right up to 12 months. What does that translate to?
iPhone 4S Wifi issues are extremely common. Judging by how common this is, I believe it's a problem caused by the manufacturing process, but can be made worse by a drop or a jolt to the phone. Which in turn cracks an inherently weak solder joint.
Often things aren't quite as bad as they seem. This type of fault only began with the iPhone 5, and exists with all other models since including the iPhone 5C, iPhone 5S, iPhone 6 and iPhone 6 Plus.