How do you get into doumbeldores office in Lego Harry Potter years 1-4?

You can't get into Dumbledore's office in Lego Harry Potter Years 1-4. There's no direct access to it in the game.

The game's story focuses on Harry's first four years at Hogwarts, and during this time, Dumbledore's office is not part of the explorable areas.

However, you can visit Dumbledore's office in the later games, Lego Harry Potter Years 5-7.