4th baby when to tell the kids?!?

Taylor
I have a 5 year old, a 4 year old, and a 17 month old and I am currently pregnant approximately 4 weeks. When should I tell my other kids? I have no history of miscarriage. With the pregnancy of my 17 month old we didn't tell the older two till we knew the sex and I'd like to include them sooner this time. Any mommas have experience with this?