L.L.
If your plan is to wait four days between introducing new foods, here is an example of what you can do.
Monday...gave cereal.
(Tue, Wed, Thur...cereal. You will have noticed by now if baby has a bad reaction to cereal, or whatever food you are giving.)
On Friday, you can introduce a new food. So say, you give bananas. Now you feed baby bananas AND cereal for Fri, Sat, Sun, and Monday. You notice baby has an upset stomach, is fussy, etc...you can assume it is the bananas, because you had no trouble with the cereal.
At the beginning of each four days, you can introduce a new food as well as continue to give baby everything you have given up to that point.