How can I route a 4" duct through a partition wall framed with 2x4s?
A 3&quot; pipe has roughly half the cross-sectional area of a 4&quot; pipe (28.3 in2 vs. 50.3). You need to maintain that size as a 4&quot; duct is already quite small. Even if you had a larger duct to begin with that's too much of a reduction.
Use the rectangular parts and/or oval ducts that meet your current flow requirements.