With WhatsApp you have one account per phone number that you activate on each phone through SMS authentication.
So if I installed WhatsApp to my iPhone 4 under number 917-555-5555, WhatsApp will then text that number with an activation code and when I enter that activation code in the WhatsApp app, it ties that number to my phone with that phone's MAC address/IMEI.
If I then upgraded to the iPhone 5 under the same number, the process repeats itself and now ties that number to the iPhone 5 with it's MAC address/IMEI. I will now lose access to WhatsApp on that number on my iPhone 4.
So if I installed WhatsApp to my iPhone 4 under number 917-555-5555, WhatsApp will then text that number with an activation code and when I enter that activation code in the WhatsApp app, it ties that number to my phone with that phone's MAC address/IMEI.
If I then upgraded to the iPhone 5 under the same number, the process repeats itself and now ties that number to the iPhone 5 with it's MAC address/IMEI. I will now lose access to WhatsApp on that number on my iPhone 4.