Mel Watts, from Australia, who blogs at The Modern Mumma, shared a snap of her daughter standing up in her crib crying alongside the letter.
“We’re sorry we lied, you aren’t our last baby,” the note read.
“You have until June 2017 to get used to the idea - until then we’d appreciate it if you learned to sleep through the night.”
The note continued: “I know this may come as a shock, feel lucky you’re the first to know out of your siblings and everyone else.
“Though we think they’ll take it better.”
Watts uploaded photo on Sunday 20 November along with the caption: “I feel like I can say morning sickness doesn’t get any better with the number of babies you have...
“What’s one more? We’re having number four.”
Watts seemed to have inspired other mums, who are hoping to have more kids in the future, with her pregnancy announcement.
“This is a class announcement,” one wrote. “Definitely stealing that if you don’t mind!”
Another wrote: “Brilliant, make sure you keep that photo for when your current youngest is 18.”