Based of the game by Candy Soft. Summary of the game:
The Hanabishi family and Hidaka family are next door neighbors.
The fathers of both households could be called best friends, so they all get along like one big happy family. In particular, Hidaka family’s only son Kouki was raised lovingly by the four Hanabishi sisters like he was their own brother.
One day, the Hanabishi and Hidaka parents were preparing for a long term overseas business trip together. Kantaro Hanabishi had only one problem holding him back from the trip; leaving his four daughters behind in Japan.
“Leave it to me. You can rely on me to watch over them!”
As a result of Kouki’s casual announcement, both parents happily departed on their trip.
“Ooh, how reliable! That was what I was waiting to hear!”
Between both of the parents, they had arranged for one of the four sisters to become Kouki’s fiance.
After Kouki’s graduation they would continue discussion of it further, but this recent event gave a chance to accelerate their plan.