I used to have an iPhone 4s which is broken now, and I got an iPhone 4 from a friend who doesn't need it anymore. I made a backup off my whatsapp chats via iCloud Drive (which doesn't exist on iOS 7), so I can't use them on the iPhone 4. By reading the Whatsapp FAQ I get the impression that this might be completely impossible. Is there any way I can have my chats on the iOS 7 device? It doesn't have to be via iCloud, and I don't mind investing some time to try a few things. Any help is appreciated.
Conventionally there is no way to do this between the devices but you can access WhatsApp chat logs via many of the various "iCloud Recovery Tools" out there. Syncios is the one that I use but it's around $50 (trial might do it for free) for a subscription but it does allow you to export those chat logs (I think to an XML off the top of my head). They're a dime a dozen those programs so you'll find one that suits you if it's the route you wish to take. They will require that Two Factor Authentication is disabled.
Sure you won't have them available on the device but you will in the short term have a copy of them for reference.