Update apps without updating ios?

I have a jailbroken iPhone 3GS with IOS 4.1 so can't update the software to IOS 6.0 in iTunes, else the phone is permanently locked. I want to download some apps including whatsapp but they keep telling me that they require IOS 4.3 or higher. How do I go about installing these apps, especially whatsapp?
any cydia tweak or something?

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