Mr. Smith had 8 kids which four of his kids being girls and 4 of his kids being boys.
The Mr. Smith had 4 daughters and Each daughter had 4 brothers is an easy to answer riddle.
Since Mr. Smith had 4 daughters which are his kids and the daughters had 4 brothers which means the brothers are also Mr. Smiths kids as well then Mr. Smith Had 8 kids.
The 4 brothers of the 4 daughters would be the sons of Mr. Smith.
This riddle seems confusing but if you sit down and really think about the answer to the riddle then it becomes very easy.
In short Mr. Smith had 8 kids which four of his kids being girls and 4 of his kids being boys.