First I'd like to congratulate the OP on his efforts to downsize his life, a challenging endeavour in our consumer society! (Voluntarily) Having less opens many doors like easier moving, quicker packing, less cleaning and maintaining...
As already said, no need to go from 4 to 1 bikes in one go. Get rid first of the easier one, and revisit after a while. You might have a clearer view of which one should be next to go.
Something that has helped me a lot to minimise is what I once read somewhere of treating eBay and the likes as my "stuff" cloud. Same as we now don't own CDs but listen to YouTube or Spotify, I don't need to own a snowboard or a tent. If I ever need them, I can rent them for that occasion or buy a used one and sell afterwards.
Once my then girlfriend was coming for a week and we wanted to do a tour. I bought a used bike in the local classifieds, did our tour, and sold the bike for the same money 2 weeks later. No need for a guest bike in my micro apartment....