The smallest bedroom at four, Privet Drive, was Dudley’s second bedroom before Harry moves in – storing all the toys he could not fit in his first bedroom, most of which were broken. Harry relocates to this room after letters start appearing at the house addressed specifically to “Mr H. Potter, The Cupboard under the Stairs.”
After Dobby uses a charm to ruin Petunia's pudding, the Dursleys lock Harry in his bedroom to prevent him from returning to school. They fit bars on the window and push meals through a cat-flap in the door. They let him out of the room twice a day to use the toilet. Ron, Fred and George rescue Harry three days into his imprisonment, using the flying Ford Anglia.