How many people can sit round a 4' diameter table?
A 4' diameter table will have a circumference of ~12.5'. Assuming each person takes up just a little bit above 2', there should be enough room for a maximum of 6 people.
Seating 120 people at round tables at wedding reception: For large round table for a seating of 8 people it would be 15 large round tables. For a smaller round table for a seating of 6 people it would be 20 tables. The general seating per round table at a reception hall is 6 people per table, but you would have to discuss the matter with the person taking care of the reception hall.