Our Dust Boot is designed to use a 5-inch intake – that is the primary reason for its effectiveness. The reducer is intended for worst-case scenarios.
Using a 4-inch when it needs a 5-inch is like giving the same amount of fuel into a Dodge Hemi engine as you would a Prius - it's not going to run well. It's roughly 56% less airflow than it needs.
The solution is to use a 5-inch duct that isn't reduced to smaller than 5-inches up until the dust boot port.