Ok thank you for reply.
Yes, S4 mini using sAMOLED and I see these splotches only on a dark background with the light turn off.
But, ok it's "normal", but it indicate a display deterioration or not? Samsung explain this problem?
Doesn't indicate anything. Your S4 Mini likely did this out of the box like so many other phone displays do. It's just the nature of the displays and you have nothing to worry about.
No phone is exactly alike so your splotches will differ from the next. You may have less than the next guy. But those are definitely normal.
Samsung Galaxy Note 3 (Verizon)