I would put some of the water in probably all of the water out of 2ft in the 4ft.
Chuck the filter and heater and fish in, and then slowly fill the tank up...
IMO theres no need to move the gravel and that there should be sufficient bacteria in your filter to handle the load that was in your 2ft just add new fish gradually so it can cope
Thats what I do anyway I just leave a filter running on a stocked tank and then move it over with new water and chuck the fish in.. Or take half the media out of an existing tanks filter..