I dont remember actually sitting the older three boys down to tell them about each others impending new sibling but when I was pregnant with DS4 I sent the boys a card in the mail. Saying I cant wait to meet you love your new brother or sister. They were all old enough to understand and it was their reactions were really cute.
I think I was about 16 weeks that time.

With DD, my brother had not long had my niece and the boys were all fascinated with her. So we waited till we had the 20wk scan to find out DD was infact a girl and I bought a little pair of pink socks that I left on the table when they got home from school.
At first they assumed they were nieces but I said no they are your sisters!!

They were over the moon and being already half way gone they didnt have to wait as long to meet her.