IPhone 4 too hot when using any application

My iphone is too hot, especially in envelope line..

Anytime when I turn on any application/battery charge, it will be really hot (a hottest point is 40 dergee ~ 104F) . And battery is too short, it spend more than 80% in only 3 hours

I am using Iphone 4 version 5.1.1 (9B206).

Please advise!