It doesn't. Consider a household with less than five members (index case + less than four others). "One in four" means that less than one additional household m

It doesn't. Consider a household with less than five members (index case + less than four others). "One in four" means that less than one additional household member will get infected on average, i.e. herd immunity at the household level is achievable. Whether that translates into herd immunity at the population level is a different question that requires information about transmission between households.