Just so you know, my TEN yr old (who was 8.5yrs when his baby brother was born) didn't notice the difference until I pointed it out to him. And then it was in the context of me telling him that circ was wrong, that I was sorry for not protecting him, and that I wanted him to be sure to never circ his own boys.
So, your 4yr old may just not notice at all - and if not, you can put off the discussion until a later date.
So, your 4yr old may just not notice at all - and if not, you can put off the discussion until a later date.