The four day wait rule with introducing foods to babies and children is something that's become very popular lately. It's a way of being able to determine if your child has an allergic reaction to a fruit or vegetable straight off the bat. So the way that you would use the four day wait rule is, for example, perhaps you're introducing strawberries to your child on Monday.
So, Monday, Tuesday, Wednesday, and Thursday you only feed your child strawberries. In these four days if your child is going to have an allergic reaction, it would materialize, either in a skin rash or their throat swelling. You will definitely know within four days if there is a problem. If you then move past strawberries and move on to zucchini, the next four days would only be zucchini. However, once your introduced a food successfully with no allergic reaction, you can definitely combine flavors. So you could introduce zucchini by mixing it with strawberries once you know that's not an issue.
But as always, it's really, really important to talk to your pediatrician, figure out a dietary introduction plan that works for you and your family, and definitely exercise caution if you have a family history of allergies.