I did this last year for the ATV4 when it was only $35/mo for the higher (highest?) tier and you only had to pay three months in advance. It was pretty nice to get an extra one for an older downstairs TV.
I'd cancel service and have my wife sign up to get this but I'm grandfathered at $35/mo and don't want to lose that. Still seems like a discount if you sign up for another account and then cancel it after four months. Maybe you could gift the service to friends or family if they already have an ATV4/4K.
Also, it's pretty hilarious that they're doing this and yet don't even support 4K yet. Knowing them it will probably be years.