Thats where the processor sits. If u open the back cover u can feel it at the back more directly behind the screen. Have u ever used a smart phone before? I ask cuz I wanted to know like compared to other devices u use how bad is it. Afaik the xperia z also has same heating up problems. Perhaps too powerful of a cpu lol. A good way to combat it is undervolting. Not sure if available for s4 d or not

No underclocking is underclocking.  Dont underclock cuz I believe our devices need the power. But I find often that the voltage being supplied to the proc is unnecesarrily high. So I undervolt a lil. Once I undervolt my devices run much much cooler. I find this especially true for samsung devices. Surfing generally produces a lot of heat cuz the processor seems to be chugging and eating up juice. So I slow down its juice intake.

if like that buy mid-end phone ok liao, high end phone then under it

thanks... i thot there is a shortcut like gingerbread, or like iphone mute switch.