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Replace the 4 inch inline fan with a 6 inch inline fan. You will go from 80 cfm to 250 cfm. In order to use your existing four inch ducting lines use a 4 inch to 6 inch adapter then add your new 6 inch inline fan. You dont need to replace your 4 inch ducting with 6 inch ducting.

Buy a 6 inch inline fan and 2 coupling adapters for 4 inches to 6 inches. Take your 4 inch ducting intake attach a 4 inch to 6 inch couplingto it and then add the 6 inch fan and then add another coupling but reverse it making it 6 inch to 4 inch back to your 4 inch ducting.