I have 3 already, who will have just turned 7, 3 and 3 when DC4 arrives.
My eldest turned 4 before her brothers were born. She totally understood about babies and had friends who had younger siblings. We read some books about it, but It was all just really easy to explain. She's always been particular verbal as well.
My younger two... I don't think they get it. They are going to turn 3 the week before the baby is due. I have told them, but I really don't feel they understand.
How do/did you prepare a 2/3 year old for a sibling? I really don't wantit to be a big shock for them, and at the moment or feels like it will be.
Am trying to avoid any major transitions when baby is due. They are now in the big boy beds. They were due to start preschool later this year, but as that week is the baby's due date we have decided to push it back to when baby will be nearly 3 months in the hope they don't feel pushed out (but I won't be juggling 3 all day when we hit crap sleep at 3/4 months!). They will still keep their nanny 2 days a week.