I didn't read the requirement of Sierra but holy crap on 4.1. Seems like 4,1 resale would be bad.
4.1 was put on Apple's Vintage and Obsolete list earlier this year ( https://support.apple.com/en-us/HT201624) . Not sure why the resale value would be hold past that event anyway. ( Not particularly surprising the OS support drops also. If the hardware is classed obsolete doing free support for it with software doesn't make much business sense also. )
As soon as they can Apple will drop the 5,1 on that list too. The gap between 2010 and 2013 is longer so the countdown clock isn't going hit year in the short term, but it is ticking. If can get away with it, it wouldn't be suprising to see Mac Pro 2012 knock the 2010 models onto the Obsolete list if Apple rationalizes it that way.