After the menstruation period, on the fourth day, it is a practice that a lady should look at her children only after partaking some type of food. This practice of taking rice or salt or tamarind on the fourth day has a scientific reason. Women have a lot of work like washing their clothes etc on the fourth day, while doing such chores they might not take food for an extended period; taking any of these will prevent acidity.
For convenience’s sake, we take raw rice or salt; that is one can take any salted item before continuing with other chores; it is difficult to cook a salty food item at that period hence this has been in practice. It is not a compulsion that one must take raw rice, tamarind, or salt, one can take any other raw food and proceed to do the chores. Though we can understand this scientifically, it is suitable to trust sampradaya and take raw rice or salt, hence this practice.