Galaxy S4 and various other products made by Samsung is configured in a choice of 'Auto adjust display tone' as the default option. To access it, you can open the Settings> My devices> Display> More settings. When enabled, this feature will adjust the brightness level based on what is displayed on the screen to save battery energy consumption.
If you have difficulty to menengarainya, there is an easy way to see the change from the use of this feature. When there are a lot of white color used in a content file, the feature 'Auto adjust screen tone' will direct the screen to save power obscures. The changes will be very subtle, but if you try to open a website via a web browser (like Google for example) and then switch to the home screen, then you will notice a decrease in the level of brightness at a glance.
This feature is definitely good if you do need the extra energy because your battery is low and you are still difficult to access sources of power in the near future. But if you want to take advantage of the screen size of the display and rich color display that can be generated (also enjoy Rp 7 million more that you spend for the Galaxy S4), then this feature you should disable.