hello plz can you help me iPhone 4S

My factory-unlocked iPhone 4, bought straight from the Apple Online Store, was delivered to me on today. I tried downloading WhatsApp from the App Store but the app couldn't be found there. I tried going through the WhatsApp site in the hope that the site would direct the download through the App Store but that didn't work either. So I then tried to sync the programme file, which was working on another iPhone, to my iPhone through iTunes but that won't work either. The transfer goes through to the phone but I keep getting an error saying "WhatsApp failed to install." Am I to conclude that WhatsApp is no longer available for the iPhone for new downloads? The WhatsApp website still has the iPhone listed as one of their supported devices but the installation just won't work. Has their policy changed? If so, is there any other free multi-platform instant messenger that I can use for contacts who are BlackBerry and Android users? I haven't any issues downloading any other apps at all and the phone works beautifully. Please someone help me with this one! Thank you.