4. A total of 52 students participated in the garden tour activities, among which 12 visited the Botanical Pavilion, 26 visited the Animal Pavilion, 23 visited the Science and Technology Pavilion, 5 visited both the Botanical Pavilion and the Animal Pavilion, 2 visited both the Botanical Pavilion and the Science and Technology Pavilion, 4 visited both the Animal Pavilion and the Science and Technology Pavilion, and 1 visited all three pavilions, then. $\qquad$ the number of students who did not visit any of these three pavilions.
【Analysis】A total of $12+26+23-5-2-4+1=51$ people visited at least one hall, so there is 1 person who did not visit any of the three halls.