Why is whatsapp not workin in iphone 3gs version 4.1, Why is whatsapp not workin in iphone 3gs version 4.1

Because whatsapp updated their program for newer iOS versions and thus dropped support for older iOS versions. That is an option of any app developer as to what legacy support they choose to offer, if any.

You need to update your iOS to use the current whatsapp version. Plug your iPhone 3Gs into iTunes and update to iOS 6.0.1

You'll have to update to iOS 5 or later, see this Apple doc -> iOS 4: Updating your device to iOS 5 or later

Then download WhatsApp again.