Why does my iphone shut the call off at 4 or so hours?

My boyfriend and I have been together for a while and sometimes we fall asleep on the phone, however, whenever we do I always wake up to the phone being disconnected. When i checked back at our phone history, it always hangs up the phone around 4 hours of talk time, even i its a full battery or its being left on the charger when we fall asleep. Is there some way to stop this?

This is likely a safety feature. Voice calls use significant CPU which heats up the phone's internal components. In order to avoid component damage or battery explosion, many battery operated electronics are equipped with this protection. I strongly suggest cutting back your conversations. Anyway, four hours on the phone is a long time, and most people would prefer spending four hours together in person anyway:-)