I think Enscheff has laid out the Bryant/Donaldson dynamic pretty well here.
The only thing I would add is that it's easy for us to focus on the "risk" of a Bryant trade, but there's quite a lot of risk in a Donaldson extension too, and not just if it goes to four years. He was great last year, but he's going to be a 34-year-old corner infielder with some recent injury problems and a big spike in strikeouts from his peak. I don't know if the data backs this up, but I'm a little leery of how hitters with a mid-range BA/lots of walks/lots of homers/lots of strikeouts profile age. On the positive side, he was still a plus defender last year (B-Ref loves his defense more than Fangraphs). If you go to four years, you do so understanding that the contract will probably look ugly by that final year - you're hoping for a couple really good years and a still-solid decline year before that. But there's certainly a chance that you give Donaldson a big money extension, and he starts declining immediately - as in, 2020.
With Bryant, at least, you're about as confident as you can be that you're going to get (at least) solid production over the next two years, which I don't think is a guarantee with Donaldson.