Mark spun the spinner 26 times and landed on yellow four times. He wants to know the probability of landing on yellow next? Is this theoretical or experimental probability? keep in mind there is 5 total sections including yellow

This would be theoretical probability. Since there are 5 total sections on the spinner and yellow appears 4 times out of 26 spins, the theoretical probability of landing on yellow is 4/26 = 2/13.