Whatsapp on my iphone 4 with ios 7.1.2 isn't working anymore. I read that whatsapp on ios 7.1.2 won't be expired. What can I do now?

I use iphone 4 for 5 years. Last week my whatsapp app announced that my whatsapp will be expired in 7 days. So then I decided to upgrade my ios 7 to ios 7.1.2. But right now my whatsapp has been expired and I can't upgrade it because my latest ios version isn't compatible