iPhone 4 usage / getting Hot

the phone will get warm but it should not harm it at all.

Before you charge it let it cool down and keep it in a cool place while charging, this is most important for battery health. the warmer it is while charging the worse it is for the battery.

 

The iPhone multi-functions as compared with the iPod Touch. When any device gets hot, in other terms if it warms up after excess use, it will start deteriorating the battery life of that device (not iPhone in particular). If it does get warm, you may encounter occasional crashes/freezing of the screen or even lags between apps. One advice is to not put the iPhone into any form of case, as this can increase heat, making it much worser.

May I ask, what were you planning on doing with the iPhone for that long to cause it to heat up? It is not a dedicated gaming console, so shouldn't be used very heavily. I wouldn't say it will stop functioning, but may cause some minor issues - one of which being decreased battery life. The cooler the device, the more battery life you will gain out of it.

To the final point, yes, it does notify you if it overheats - this is particular when in outdoors in sunny weather or under warm conditions. The iPhone will display a warning box and switch off automatically. My advice, do your gaming on your Touch, and handle other stuff (such as emails, web, phone/messages) with your phone.