Can you turn a Verizon iPhone 4 into prepaid?

Yes you can - simply speak to your network provider. Assuming your contract has passed its minimum length, it's simply a case of changing the accout from a monthly fee to a prepaid one. Your network provider will issue a final bill from your monthly contract, and should issue you with a 'top-up' card - or you can use a debit/credit card to add money to your phone's account.