I did a 4" to 3" reducer with 3" ducting and it worked fine, not optimally because of the backpressure that the reducer produced, but it worked. Thing is, I had to use a 130cfm fan to vent a 16 cuft box (2x2x4) because of that backpressure. With the size of area you want to use, I don't think a single 4" fan is going to cut it honestly as they don't really get any higher than 180cfm that I've seen.