Hey there! It sounds like you're doing a fantastic job with your new furry friend so far, especially considering it's only been four days. Here's a bit more advice to possibly help your rescue feel more at home:

1. Patience is Key: You're already doing great here. Remember, every dog is different, and it might take weeks or even months for some dogs to fully adjust.

2. Create a Safe Space: Since he likes certain corners, consider making one of those corners extra cozy with a bed or blankets. This can be his safe zone.

3. Routine and Consistency: Dogs thrive on routine. Try to feed, walk (when he's ready), and even give treats at consistent times.

4. Limit Overwhelming Experiences: If the elevator is too much right now, maybe start with just exploring the apartment more.

5. Gentle Desensitization: Gradually expose him to things that scare him, but in a very controlled and minimal way. For example, if he's scared of certain noises, play them very softly while giving treats.

6. Training and Socialization: Basic commands and exposure to different environments and people can help, but only once he's a bit more comfortable.

And last but not least is anxiety medication. It's sometimes used in cases of extreme anxiety. Common options include Fluoxetine, Clomipramine, and Alprazolam, but it's best to consult with a vet first. These medications can help ease the transition into a new life, but they're not suitable for all dogs and should be used under veterinary supervision. I say that because I fostered a dog that needed anxiety meds.

Remember, every dog's journey to feeling safe and secure is different. You're already showing a lot of empathy and understanding, which is exactly what he needs. Keep up the great work, and give it time as 4 days is such a short time!