It’s a rare situation to my knowledge but popular and high standards restaurants will deny you booking a table for 2 if they only have tables of 4 available. They do this because they have faith that they will almost certainly fill that table of 4 and will take 4 customers over 2.
Its kinda null that if you ring up several days in advance to book a table for 2 and they say we can only do bookings for 4, cant the restaurant re-arrange? Or cant I just say a booking for 4 anyway? its pretty ridiculous but it does happen.
The only business conditions that allow a restaurant or eatery to reject a booking for 2 and only take bookings for 4+ is if they are doing extremely well and a popular choice for diners.
They are not afraid and know they will not have any problems with filling a larger table. The market in the area favors this place, they are a big player which is highly regarded.
Can consumers do anything about it? Nope its a quiet simple no, just dine elsewhere. Bad luck on missing out on the limited smaller tables at this restaurant, try elsewhere.
Whilst it is seen as greedy and perhaps unethical or slightly discriminating, denying a booking based on limiting your capacity is perfectly legal.