My iPhone 3G version 4.2 won't download Whatsapp?

I'm trying to download Whatsapp, Viber, Heytell etc on my friend's iPhone 3G, but it says 'Update required. This app requires iOS 4.3'.
She has version 4.2.1. Before she upgraded her iOS (I don't know what version she had, but it was definitely older than her current one), she had Whatsapp, Viber and all the other apps which are now not downloading. So it seems very strange.
Your assistance is much appreciated!

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You can find specific instructions on how to download Viber for your iphone at: http://viber.kayako.com/Knowledgebase/Article/View/3/4/setup-viber-for-your-iphone

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