Really? You want to go THERE??!?!?
The iPhone 4s had a 3.5" screen with 3G.
The iPhone 5 has a 4" screen with 4G LTE.
That alone is a bigger difference than the GS3 to the GS4.
A non tech person can't see that! I OWN a 4S and I can barely tell a difference between it and a 5 if both are wearing cases.
My 4S has near the same outside diameter in a lifeproof case as a 5 in a commuter series Otterbox. I use that example because the Otterbox is so popular and has similar dimensions as many cases out there.
I'm not going to say that's different between the S3 and S4 because with a case they will be equally hard to tell apart maybe harder.
Point is a .5" bigger screen diagonally so less vertically will be unnoticeable to someone that either isn't looking for it and/or has never used an iPhone.
It might be easy for you to see being an iPhone user I'm sure. In the same note it will be easy for an S3 owner to notice a bigger screen with a smaller bezel on an S4.