Can I put a 4 inch haet duct in one of my cold air returns. I just don't see any other way without getting rid of the duct for remodeling purposes. ANY help would greatly be appreciated.

Thanks

If you do put the 4-inch duct into the return, how will the air be supplied? If it comes out right at the return grille, the air will short-cycle and never condition the space...in other words it will come out of the supply and be drawn right into the return.

Also, you will be reducing the cross-sectional area of the return duct, and the area of the return grille, and both could lead to more noise as the same amount of air will have to pass through smaller spaces.