Consider the BK Princess with a 2.8 cu ft firebox. That's four times larger than a Squirrel firebox, and yet it uses the same size 6 inch flue. The Squirrel comes from the factory with a 6 inch collar, but from operating it I believe it would work just fine with a reducer, a 4 inch flue, and with the internal restrictor removed.
I'd email Morso and ask them. They'll know for sure.
Greg
I think the manual is pretty specific about the 6" . I'm sure the BK princess is an impressive stove, but my room is so small and segmented from the rest of the house I'm concerned the squirrel might even be too powerful! Haha