Are the duplexes in the same area and of the same quality of the subject property? From what you post, you live in rural Wisconsin. These two questions would be big factors in the puzzle.
Yes they are in the same area, they are better quality then the subject.
Based on the fact that smaller tracts of land or investment properties tend to sell for a higher price/SF or unit than larger tracts/units due to sheer magnitude of the investment you might be correct. However, investors in smaller rental properties tend to be less educated than investors in larger properties. Therefore the use of 2-unit properties could very well produce a lower value depending on the specific market.
Another factor to consider is vacancy. If you own a four unit property and have one vacancy for an extended period of time you are losing 25% of your income. With a two unit property you have 50% vacancy, and might have a problem paying the mortgage.
Debt ratios might also be considered, and affect a cap rate (or what a banker will loan, and in what terms) in some properties. What is the loan rate on a duplex vs a four or six unit property? Significant, I would guess depending on the credit rating of the borrower and a credible appraisal.
These are things you need to consider, but of course I am playing devil's advocate.
Understand
Nothing is black and white in appraising.