You don't get to meet four Princesses, you only get to meet two Princesses at a time. You enter the queue and choose which two (2) Princesses you want to meet. From the Disney Parks Blog post by Ms. Fickley-Baker;
"As a reminder, Princess Fairytale Hall will offer two separate character greeting experiences, which means there are two queues to choose from. You can choose to meet either Cinderella or Rapunzel, each of whom will be joined by one fellow Disney princess."
Then you exit the facility. If you want to meet the other two Princesses on the other side of the wall that you didn't see, you either get back into the Standby line or get another Fastpass for later in the day and go again, and this time you choose the other side of the room to see the other two gals.
You do have the ability to choose which two Princesses you will see when you get your Fastpass. From Ms. Fickley-Baker again;
"Yes, there will be two separate FASTPASS options for guests to choose from: Rapunzel and a princess friend or Cinderella and a princess friend. (Also, the FASTPASS distribution for Princess Fairytale Hall will be located near Peter Pan’s Flight)."
So, two Princesses at a time. Then exit and go again some other time for the other two you want to see.
Interesting that this is being operated very differently from the Princess Fantasy Faire and the Royal Hall meet n' greet pavilion there. Very differently.