Q: When I am charging for iphone 4, it always very hot, why?

Hello Kczn1234,

Depending on the warmth of the iPhone, this can definitely be normal behavior. The following article explains warmth while charging and using other functions more in-depth:

iPhone, iPad, and iPod touch (4th generation): Keeping device within acceptable operating temperatures

http://support.apple.com/kb/HT2101

When using the device or charging the battery, it is normal for it to get warm. The exterior of the device functions as a cooling surface that transfers heat from inside the device to the cooler air outside.

Let's not forget your iPhone will warn you if it is over heating, and if it needs to, it will shut itself down. As long as you have not experienced this warning, what you are sensing may be normal. By all means, if you believe the temperature is a concern, you can isolate the issue by restoring your iPhone as new:

iTunes: Restoring iOS software

http://support.apple.com/kb/HT1414

Thanks,

Matt M.