Not sure about the trade in, but the best way to go about it is to buy the phone up full price at t-mobile.com. You have to choose one of their plans, I think the $40 plan would be the cheapest. Once you receive the phone, call them asap and tell them you are not interested in their services and not to activate your account.
That's what I'll do. Buy the Note 4 from T-mobile. $750 + $52 for taxes and $40 for the first month service if they ask me for it. Receive the phone and just install my MetroPCS sim card in. Since MetroPCS runs on T-mobile's network, I won't have to get it unlocked. Otherwise, you need to use their services for 40 days and have the account in good standing before they unlock your phone. Even if you pay up front for the phone