Having problems restoring whatsapp messages on new iphone when I have kept my old number?

I have just bought an iPhone 6s. Previous to this I had an iPhone 4. I've kept the same number. I've been using whatsapp for the last 3-4 years on my iPhone 4 and have lots of messages stored in it. I backed up my iPhone on to iTunes and then restored the backup for my new phone. Everything is on there apart from my whatsapp messages. How do I get these messages from my old phone to me new phone?

Any help will be appreciated.

Transfer the backup file, it may work and if you want to secure you device try Leo Privacy Guard. It's the best app in the Play Store.