I CANNOT for the life of me understand WHY screw maps for the iPhone 4s (or all models for that matter) don't have the corresponding size written beside each hole, and why they are not colour-coded to make life just a 'wee' bit easier???
I have a digital vernier caliper and it is STILL very difficult to determine the minute differences between the screws!!!
If you tear down a phone, have a magnetic/holed screw mat, and can follow a video, then chances are you will be okay, because you have the screws coming out of the phone and are able to put them directly into the appropriate hole.
However.... if you are unlucky (or just plain dumb like me!), and didn't have the screws in a magnetic holder, and the cat happens to jump up on the paper with the screws on it....well....then you are in serious trouble!
Just TRY to determine the size of the screws!!! It is a painstaking task- even WITH a digital caliper.
Yes, the ifixit video tells you what size screw goes where....but it is MEASURING the screws that makes this so difficult. I am dreading trying to reassemble my phone.