How do you work out the code for the fridge in the basement?

They don't explain the whole puzzle:

469 - 346 = 123

366 - 132 = 234

589 - 244 = 345

??? - 111 = ???

The difference between the results of subtraction is always 111: 234 -123 = 111, 345 - 234= 111... etc. so, add 111 to 345 = 456, now you have:

??? -111 = 456 ie 456+111= 567

You should have a paper with a clue. Look at the differences in the 2 numbers in each column. Check the guide for Part 4 for the method and the solution.