I have 4 as well. Single speed road bike, mtb with fenders, track style bike with gears, and another mtb setup as a road bike. They are different enough that I can justify having them.
My "road bike" pretty much does everything and is my go to bike. It is also the lightest and fastest, but I don't get it wet or dirty.
Single speed I use for commuting on dry days.
Mtb with fenders I use during winter, when its wet and dirty, or I plan on exploring unpaved surfaces.
The Track bike I honestly don't need. I built it primarily because I liked the way it looks. It gets ridden the least.
I still look at other bikes for sale all the time. Kind of want a novelty bike like a mini velo.