Do not listen to these guys bro,your contract with Verizon is not on the phone,its on the service.You can use any phone you want and trading,selling or smashing your phone will not touch your contract.
Right now you can buy a used gs4 off craigslist and walk into a Verizon store and say I need a sim made for a gs4 with my number and they will do it for FREE and 5 min after you pop the new sim in your gs4 your online account with Verizon will show a gs4 instead of an iphone 4.
let me repeat this,your contract is not on the hardware its the service you need to have for 2 years.If your iphone 4 gets stolen you can use a cheap old phone you have or buy a new phone full retail and use it.
trade your phone up!!!
you can buy a unlocked gs4 at Verizon at full retail and then put in the iemi number on the Samsung site and they will mail you a 200 gift card for getting an gs4