He's stuck behind Brady, other teams know this so his value is lessened. He might fetch what they put into him, but I doubt it. Hence the 4th rounder. If you want a comparable guy, you are looking at Mike Glennon. He's a 3rd round guy, maybe lower, but you get roughly the same thing as you would with Mallett. So much so that if Mallett is traded, some mocks have NE picking him up @ 91. He's raw, but greater upside, similar size, without issues in the past, and a couple of years younger as well. I don't see moving a 2nd when you can grab a similar cheaper guy with a lower pick. Even with improved mechanics and clipboard technique of Mallett, you are still paying for potential, not actual production so you are not gaining value.
Rodgers was a first rounder. He is/was the future, so it makes sense that they invested everything into him to get him to where they wanted him. Mallett's future is not that certain.