Final answer:
Diarrhea is an immediate cause of undernutrition for the child in question, as it affects nutrient retention and absorption, leading to malnutrition.
Step-by-step explanation:
According to the UNICEF framework, diarrhea is an immediate cause of undernutrition for the 4-year-old boy in Bangladesh. This is because infectious diseases like diarrhea directly affect a child's ability to retain and absorb vital nutrients, thus contributing to malnutrition. Both undernutrition and infectious diseases have a synergistic relationship where each exacerbates the severity of the other, which is particularly dangerous for children who are in critical stages of growth and development.