easiest way to transfer whatsapp chat history from note 3 to note 4 topic

Guys pls help i was about to buy the note 4 but the people at the 2 shops I tried didn't know how to transfer them from the note 3 they said they tried before but it didn't work for their earlier customers. Can u please teach the best way to transfer them especially that I heard whatsapp was pre-installed in the note 4 already thanks thanks