Can I get a refund if I book a hotel in Vienna for 4 days and stay for 3 days

generally speaking, you can't. even if you return it, your deposit will be deducted.

when you checked in, it was already four days.

I gave you the room, but the hotel didn't put it outside to make a reservation.

so if you check out temporarily at this time, you will suffer losses for the hotel.

or he will deduct your deposit, so it's definitely not cost-effective to check out.

if you ordered it online, you can check the usage rules.

if it's clearly stated that it can't be returned, it can't be returned.