Check on Wednesday for Clean Days

I went to the rabbi and told him that I have great difficulty with the checks during the seven clean days, and he told me to check on the first, fourth, and seventh days of cleanliness. What is the significance of checking on the fourth day?

There is a halacha that if more than five days pass between the first and last check, the counting of the seven clean days must start anew. Therefore, a woman who checks on the first and seventh days of cleanliness fulfills her obligation. However, there is a concern that she might delay on the seventh day and not check during the day, and then she will not be able to check the next day and immerse. Therefore, if there are difficulties with the checks, but there is an opportunity to make an extra effort for an additional check, a good advice is to check also on the third or fourth day of cleanliness, so that if you forget or there is some reason not to check on the seventh day, you can check the next day and immerse, and you will not need to count all seven days from the beginning.

Shulchan Aruch Yoreh De'ah 196