I agree 4 years and ~$50-$55M is reasonable. To everyone that thinks this is low ball, need to keep in mind we already have 2 years at ~15-$20M.
So in effect we would be buying 2 years at ~$35M, but there is some risk for Twins and increased security for Sano.
If his hitting declines some and he can't play 3rd base in a couple years, or has a serious injury, then he won't get $$35M from anyone for his first two free agency years.