Samsung Galaxy S4 upper half of the screen is slightly darker

[PresidentORO]

A few days ago, I bought secondhand Samsung Galaxy S4 I9505. I noticed that on the upper half of the screen is big 'square'
(that square is 90 % of the upper half ) that is slightly darker than the rest of the screen.I made screenshot of the screen,checked it on my computer and there was not 'square'.I'm not sure if that is hardware or software problem.Please help.

 

[Petr Tanker]

Try make some test of of your screen.