I thought about this about a week ago, but you take the cake for involving 3 carriers (and a 4th carrier if you count the fact that your old iPhone 4 is an AT&T phone). Because I'm a stickler for details, I feel compelled to comment on some things...
I figured out a plan to get a free iPhone 6. It uses loop holes but it's not technically dishonest or illegal.
Illegal, no. Dishonest? That's debatable. Your plan leaves Verizon losing a little bit of money, and T-Mobile losing several hundred dollars worth of money.
So this isn't technically free but much cheaper than buying a phone outright.
Thread fail. Your thread is titled "Get a Free iPhone 6". ;-P
OK, so a few more comments on the details:
1) Good luck finding an iPhone 6 at a Verizon store anytime soon.
2) You will need to submit your *final* Verizon bill, showing the ETF, to T-Mobile. If you attempt to cancel Verizon within the first 14 days, expect them to hassle you about getting that iPhone back, so you should plan to cancel after 14 days. Plus, T-Mobile might find it fishy if they saw that your 1st bill and your final bill were one and the same and might suspect that you're up to something (i.e., fraud).
3) So you'll need to keep that Verizon line active for 14 days, then sign up with T-Mobile. Not sure if T-Mobile handles canceling your account with Verizon, or if you need to circle back and do that. Probably the latter. So now you can cancel with Verizon so that the ETF date is >= the T-Mobile activation date. And now you have to wait for that final Verizon bill showing the ETF fee. Depending on how the billing cycle runs, that could be another 30 days. Not sure how proration works, but I'd plan to pay at least one month of service of Verizon.
4) And now the new T-Mobile contract is active and you're starting to pay them, too. The new Verizon bill showing the ETF fee might take a while to show up online (depending on how the billing cycle runs, maybe up to 30 days from the time you canceled?). So you could be looking at paying for a full month of T-Mobile service before you can even submit that request for ETF reimbursement to T-Mobile. Then, you are looking at waiting up to 8 weeks before the card arrives. I wouldn't expect it to arrive early. So now we could be looking at 3 months of active T-Mobile service before you can cancel with them. And depending on how T-Mobile's billing cycle and proration works, it's not beyond the realm of possibility that you somehow end up getting billed for a 4th month. But I'll leave that out of the final numbers.
So, all told, you could easily be looking at 1 month of Verizon service and 3 months of T-Mobile service. Plus lots of taxes and such. And an *AWFUL* lot of hassle. And in your situation, you'll also be looking to trade that phone, so add in all of that hassle. The Verizon iPhone is missing a few Sprint-specific bands, so if you want to end up staying with Sprint, you'll definitely be needing to trade it (or resell it and buy a Sprint iPhone 6 at full price).
The general plan makes a lot more financial sense in other scenarios. For example, I'm on AT&T with an old family plan. Let's say I have 3 lines of service, all available for upgrades. Let's say I actually *want* to move to T-Mobile (or Sprint). So, I re-up with AT&T for 2 more years, buy 3 new iPhone 6's (at $200 each, minus any trade-ins I have), and then I run right over to T-Mobile/Sprint, switch to them, and let them pay off my ETFs. I just got 3 new iPhones for $600 total, free and clear.
Slight variation on the above: Maybe I'm happy with AT&T but want to switch to their mobile share plan. I do the above, suffer through T-Mobile/Sprint's poor coverage in my area while I wait for my ETF reimbursement to arrive, and then cancel with them and switch back to AT&T on their newer mobile share plans and bring my own AT&T iPhone 6's back with me. That assumes that AT&T hasn't put some sort of fraud alert on my account, such that they won't take me back.