Yes, I think a four-year contract would be fantastic. That would be between double and four times many of the contracts that I had in Canada. It would be competitive with winning fellowships elsewhere. For example, in Australia, a grant might be three years or an investigator award is five, so a four-year contract would be very competitive.
As for a tax deduction of student debt, I think that people would probably be quite happy with that. I don't have any personal experience to relate to that, but that kind of incentive would definitely encourage people to stay in the field.