one is a temporary storage for my gyroids (until I get a set of four matching ones for a room in one of my houses)
then I have a planetarium (star projector in the middle, tons of box sofas, a mannequin presenter, and various space displays in the corner)
a portrait gallery (villager pics)
and unorderables that I intend to trade for stuff I actually want
I'm using one room to display all the Golden furniture I've been collecting. Another room I'm using to store furniture customized with ore, but I doubt that will be its permanent purpose.
The other two are just for storage right now. I like the idea of turning one into a small cafe or restaurant, and another for villager pictures.
1 is all regional items and some unorderable DLC(Its complete at the time being :3)
2 is a complete wetsuit exhibit
3 is a 7-11 display
4 is the Zodiac set in nice glass cases with the lunar wall and flooring
basically I just use it to store/show off rare items haha