Four is the number of people who will ever fit in this kitchen, and the number of days a mug survives before I wash it.
I used to own nineteen mugs. I know because I counted them during the move, sitting on the floor with a roll of newspaper and a growing sense of shame.
The audit
Eleven had logos on them, from jobs I had left and conferences I had not enjoyed. Three were gifts I felt too guilty to give away. Two had chips I kept meaning to do something about. One said Greatest Aunt, and I am nobody’s aunt.
That left two mugs I actually liked, and I reached for those every single day while the other seventeen stood behind them like understudies.

Why four
Two feels like a statement. Six feels like you are expecting a committee. Four means that if friends come over you can make everybody tea, and if nobody comes over you have a day or two of grace before the washing up becomes a moral issue.
A cupboard should hold what you use, and nothing you are saving for a better version of yourself.
What it did to the rest of the house
I wish I could say I stopped there. I did not. The mugs led to the glasses, the glasses led to the drawer with the takeaway menus, and by spring I had a kitchen where I could find a spoon on the first try.
I am not a minimalist. There are still forty books on the stairs. But I no longer drink my coffee from a souvenir of a software migration, and I count that as a life well organised.