I've been living in Spain for the last couple of years with normal 1 year leases. This year however I didn't know if I would be able to commit to another year so I started looking for multi-month AirBnB rentals. My current 1 year lease is 920€/month (with utilities about 1000€) for a furnished 2 bedroom apartment with a great view of the beach. All of the AirBnB's I was looking at even with the ~50% monthly discount were showing around 1700€/month. I picked 3 I liked and messaged them through AirBnB that I was living in the area and I would like to check out the apartment in person but I can only afford 900€/month. All 3 replied to me with counter offers between 1100€ and 1300€. I picked the one I liked best, met it person, and we agreed on 1000€/month for a 3.5 month lease including all utilities. We signed a lease contract on the side and both avoided the fees.
AirBnB is great for booking a place when you are not in the city but I think you can get much better deals if you know the local market and are able to meet the landlords face to face.
TLDR: If I was going to look for a place for 3-4 months I would book the first month through AirBnB then apartment hunt in person in the target city for the remaining months at a steep discount.