I did a few trades in for Verizon 6 using iPhone 4 already. So I hope to clarify some points for you:
1/ You can either upgrade, sign new contract OR even get the iPhone 6 outright without contract
Here the quote from their website "You will receive a $200 VZW Trade-in Gift Card when you trade in your old iPhone 4, iPhone 4s, iPhone 5, or iPhone 5c, and purchase a new iPhone 6 or iPhone 6 Plus with a new Verizon Edge agreement, 2 year contract, or when you pay full retail price."
2/ You can do it in-store,
I called and confirm with the rep at Recycling Department and did 4 trades in-store. They gave me 4 gift cards (emailed to you instantly).
However, there's a store I got denied and asked to do it online.
Please insist on the policy or call the Recycling Department Rep right there to force them to make the trade for you
3/ For those that can find the iphone 6 you want instore, they will instantly give you $200 of the purchase price, ie you can trade your iphone 4 for their iphone 6 with $0 out of pocket.
4/ You MAY buy from other retailers and successfully trade in
The rep that did the trade for me only look at the ESN of my new Verizon iPhone 6.
Few of my new iPhones 6 not even activated yet.
So I best advice is to walk to the store, say vaguely that you just got the iPhone 6 and ask to trade your iphone 4.
Show them the ESN on the iPhone 6. AFAIK, The store rep cannot check where the phone was purchased.
5/ Water damaged iPhone 4 that still functions properly and no screen crack still worth $200
Don't believe me? Go to Verizon trade in and click Yes on the water damage question (that's what the rep also do, click on those questionnaire to determine value).
The result value is still $200