Well based on the examples posted, it does look like it can be a legitimate problem on effected units. Also strange if its affecting 4s more than 3s (I definitely cannot get anything even remotely similar to occur on my 3, and I was under the impression display tech was the one thing that hadn't really changed this round).
That said, you can pretty much look to any apple product release and find a display quality complaint. The 3s lead to many threads here about uneven backlighting and colour temperatures, and I ended up exchanging mine after a week (luckily the replacement has been absolutely great--no complaints).
My feeling is that apple does have market leading display tech, but they also have incredibly high volume, which makes consistency and quality control much more challenging. Even if only 1% of their product has a problem, that can easily equate to hundreds of thousands of actual units. (And if it is a "bad batch" scenario, it makes sense that if you come across a bad unit, you might come across a bunch more if that batch was shipped to your region). It's not really an excuse or a condemnation, it's just the reality of being such a big operation.
Hopefully those affected can get themselves a unit that they're happy with!