Can be achieved by placing 4 different Diner style furniture pieces, it doesn't matter what type (can be kitchen, chairs, art) as long as it has the Diner indicator in the game.
You can unlock the Diner style furniture by researching one of the furniture styles at the Shrine.
The game will state if you already have 1 of an item and that the stats won't go up, i.e.; have 2 diner chairs of Diner: 2 stat, is not 4 stat, it's 2 stat.