To open the door in Deltarune Chapter 4 , the most likely solution is the Mike door password: 6453. That code was reported in multiple walkthrough-style sources as the key for the first locked Mike door in Castle Town.

What to do

  1. Go to the TV building in Castle Town.
  2. Interact with the Mike door.
  3. Enter 6453.

Second door

There’s also a second Mike-related door later on. One guide says you need to pat the cat statues , save the Mause to get the required hand ability, and then use the microphone mechanic to make enough sound for the door to open.

Extra note

A separate walkthrough mentions a different secret-door sequence in Chapter 4 involving notes and a piano , which opens a passage in the old man’s study, so “the door” may refer to a different puzzle depending on where you are.

If you mean a specific door location, I can narrow it down to that exact room.